In selector check, prefix of reference must match import qualifier (scala#20894)

This PR changes the `CheckUnused` phase to rely on the `MiniPhase` API
(instead of custom traversal). That improves fidelity to `Context`
(instead of approximate scoping).

The phase should work seamlessly with subsequent linting phases
(currently, `CheckShadowed`).

It is a goal of the PR to eliminate false reports. It is also a goal not
to regress previous work on efficiency.

A remaining limitation of the current approach is that contexts don't
provide a nesting level. Practically, this means that for a wildcard
import nested below a higher precedence named import, the wildcard is
deemed "unused". (A more general tool for "managing" or "formatting"
imports could do more to pick a preferred scope.)

This PR adds `-Wunused:patvars`, as forward-ported from Scala 2: it
relies on attachments for some details about desugaring, but otherwise
uses positions (where only the original patvar has a non-synthetic
position).

As in Scala 2, it does not warn about patvars with the "canonical" name
of a case class element (this is complicated by type tests and the
quotes API); other exclusions are to be ported, such as "name derived
from the match selector".

Support is added for `-Wconf:origin=full.path.selector`, as in Scala 2.
That allows, for example:
```
-Wconf:origin=scala.util.chaining.given:s
```
to exclude certain blessed imports from warnings, or to work around
false positives (should they arise).

Support is added to `-rewrite` unused imports. There are no options to
"format"; instead, textual deletions preserve existing formatting,
except that blank lines are removed and braces removed when there is
only one selector.

Notable fixes are to support `compiletime` and `inline`; there are more
fixes to pursue in this area.

Fixes scala#19657
Fixes scala#20520
Fixes scala#19998
Fixes scala#18313
Fixes scala#17371
Fixes scala#18708
Fixes scala#21917
Fixes scala#21420
Fixes scala#20951
Fixes scala#19252
Fixes scala#18289
Fixes scala#17667
Fixes scala#17252
Fixes scala#21807
Fixes scala#17753
Fixes scala#17318
Fixes scala#18564
Fixes scala#22376
Fixes scala#21525

Author: sjrd

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -818,7 +818,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
 
       methSymbol  = dd.symbol
       jMethodName = methSymbol.javaSimpleName
-      returnType  = asmMethodType(dd.symbol).returnType
+      returnType  = asmMethodType(methSymbol).returnType
       isMethSymStaticCtor = methSymbol.isStaticConstructor
 
       resetMethodBookkeeping(dd)
@@ -915,7 +915,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
             for (p <- params) { emitLocalVarScope(p.symbol, veryFirstProgramPoint, onePastLastProgramPoint, force = true) }
           }
 
-          if (isMethSymStaticCtor) { appendToStaticCtor(dd) }
+          if (isMethSymStaticCtor) { appendToStaticCtor() }
         } // end of emitNormalMethodBody()
 
         lineNumber(rhs)
@@ -936,7 +936,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
      *
      *  TODO document, explain interplay with `fabricateStaticInitAndroid()`
      */
-    private def appendToStaticCtor(dd: DefDef): Unit = {
+    private def appendToStaticCtor(): Unit = {
 
       def insertBefore(
             location: asm.tree.AbstractInsnNode,

compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala

@@ -12,10 +12,10 @@ import java.util.zip.{CRC32, Deflater, ZipEntry, ZipOutputStream}
 
 import dotty.tools.dotc.core.Contexts.*
 import dotty.tools.dotc.core.Decorators.em
+import dotty.tools.dotc.util.chaining.*
 import dotty.tools.io.{AbstractFile, PlainFile, VirtualFile}
 import dotty.tools.io.PlainFile.toPlainFile
 import BTypes.InternalName
-import scala.util.chaining.*
 import dotty.tools.io.JarArchive
 
 import scala.language.unsafeNulls