Add help for potentially missing crate in Cargo.toml

On resolve errors where there might be a missing crate, mention `cargo add foo`:

```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
  --> $DIR/conflicting-impl-with-err.rs:4:11
   |
LL | impl From<nope::Thing> for Error {
   |           ^^^^ use of unresolved module or unlinked crate `nope`
   |
   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
```

Author: estebank

compiler/rustc_resolve/src/diagnostics.rs

@@ -811,7 +811,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     }
                     err.multipart_suggestion(msg, suggestions, applicability);
                 }
-
                 if let Some(ModuleOrUniformRoot::Module(module)) = module
                     && let Some(module) = module.opt_def_id()
                     && let Some(segment) = segment
@@ -2037,7 +2036,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             self.current_crate_outer_attr_insert_span,
                             format!("extern crate {ident};\n"),
                         )],
-                        format!("consider importing the `{ident}` crate"),
+                        format!(
+                            "if you wanted to use a crate named `{ident}`, use `cargo add {ident}` \
+                            to add it to your `Cargo.toml` and import it in your code",
+                        ),
                         Applicability::MaybeIncorrect,
                     )),
                 )
@@ -2216,6 +2218,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 let descr = binding.res().descr();
                 (format!("{descr} `{ident}` is not a crate or module"), suggestion)
             } else {
+                let suggestion = suggestion.or(Some((
+                    vec![],
+                    format!(
+                        "if you wanted to use a crate named `{ident}`, use `cargo add {ident}` to \
+                         add it to your `Cargo.toml`",
+                    ),
+                    Applicability::MaybeIncorrect,
+                )));
                 (format!("use of unresolved module or unlinked crate `{ident}`"), suggestion)
             }
         }

tests/rustdoc-ui/ice-unresolved-import-100241.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `inner`
 LL |     pub use inner::S;
    |             ^^^^^ use of unresolved module or unlinked crate `inner`
    |
-help: consider importing the `inner` crate
+help: if you wanted to use a crate named `inner`, use `cargo add inner` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate inner;
    |

tests/rustdoc-ui/intra-doc/unresolved-import-recovery.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `unr
 LL | use unresolved_crate::module::Name;
    |     ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved_crate`
    |
-help: consider importing the `unresolved_crate` crate
+help: if you wanted to use a crate named `unresolved_crate`, use `cargo add unresolved_crate` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate unresolved_crate;
    |

tests/rustdoc-ui/issues/issue-61732.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `r#m
 LL | pub(in crate::r#mod) fn main() {}
    |               ^^^^^ use of unresolved module or unlinked crate `r#mod`
    |
-help: consider importing the `r#mod` crate
+help: if you wanted to use a crate named `r#mod`, use `cargo add r#mod` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate r#mod;
    |

tests/ui/attributes/field-attributes-vis-unresolved.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `non
 LL |     pub(in nonexistent) field: u8
    |            ^^^^^^^^^^^ use of unresolved module or unlinked crate `nonexistent`
    |
-help: consider importing the `nonexistent` crate
+help: if you wanted to use a crate named `nonexistent`, use `cargo add nonexistent` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate nonexistent;
    |
@@ -15,7 +15,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `non
 LL |     pub(in nonexistent) u8
    |            ^^^^^^^^^^^ use of unresolved module or unlinked crate `nonexistent`
    |
-help: consider importing the `nonexistent` crate
+help: if you wanted to use a crate named `nonexistent`, use `cargo add nonexistent` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate nonexistent;
    |

tests/ui/coherence/conflicting-impl-with-err.stderr

@@ -3,12 +3,16 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nop
    |
 LL | impl From<nope::Thing> for Error {
    |           ^^^^ use of unresolved module or unlinked crate `nope`
+   |
+   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
 
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
   --> $DIR/conflicting-impl-with-err.rs:5:16
    |
 LL |     fn from(_: nope::Thing) -> Self {
    |                ^^^^ use of unresolved module or unlinked crate `nope`
+   |
+   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
 
 error: aborting due to 2 previous errors
 

tests/ui/delegation/bad-resolve.stderr

@@ -86,6 +86,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `unr
    |
 LL | reuse unresolved_prefix::{a, b, c};
    |       ^^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved_prefix`
+   |
+   = help: if you wanted to use a crate named `unresolved_prefix`, use `cargo add unresolved_prefix` to add it to your `Cargo.toml`
 
 error[E0433]: failed to resolve: `crate` in paths can only be used in start position
   --> $DIR/bad-resolve.rs:44:29

tests/ui/error-codes/E0432.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `something`
 LL | use something::Foo;
    |     ^^^^^^^^^ use of unresolved module or unlinked crate `something`
    |
-help: consider importing the `something` crate
+help: if you wanted to use a crate named `something`, use `cargo add something` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate something;
    |

tests/ui/extern-flag/multiple-opts.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `som
    |
 LL |     somedep::somefun();
    |     ^^^^^^^ use of unresolved module or unlinked crate `somedep`
+   |
+   = help: if you wanted to use a crate named `somedep`, use `cargo add somedep` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error
 

tests/ui/extern-flag/noprelude.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `som
    |
 LL |     somedep::somefun();
    |     ^^^^^^^ use of unresolved module or unlinked crate `somedep`
+   |
+   = help: if you wanted to use a crate named `somedep`, use `cargo add somedep` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error