group internal/... import paths as standard library

The Go standard library includes internal packages such as
internal/testenv, and even though such packages cannot be imported
directly by users, they are still imported by std packages like os
and should be grouped with the rest of the std packages.

I can't recall what my logic was for excluding internal/ prefixes
from std grouping, but the logic no longer makes any sense to me.
This makes gofumpt less grumpy with how Go groups its own std imports.

Fixes #307.

format/format.go

@@ -978,10 +978,8 @@ func (f *fumpter) joinStdImports(d *ast.GenDecl) {
 		case periodIndex > 0 && (slashIndex == -1 || periodIndex < slashIndex),
 
 			// "test" and "example" are reserved as per golang.org/issue/37641.
-			// "internal" is unreachable.
 			strings.HasPrefix(path, "test/"),
 			strings.HasPrefix(path, "example/"),
-			strings.HasPrefix(path, "internal/"),
 
 			// See if we match modulePrefix; see its documentation above.
 			// We match either exactly or with a slash suffix,

testdata/script/std-imports.txtar

@@ -160,10 +160,11 @@ import (
 // All of the extra imports below are known to not belong in std.
 // For example/ and test/, see https://golang.org/issue/37641.
 import (
+	"internal/bar"
 	"io"
 
 	"example/foo"
-	"internal/bar"
+
 	"test/baz"
 )