Fix undercompilation upon ctor change (#19911)

Fixes #19910
Fixes sbt/zinc#1334

## Problem
Scala 3 compiler registers special `zincMangledName` for constructors
for the purpose of incremental compilation. Currently the
`zincMangledName` contains the package name, which does not match the
use site tracking,
thereby causing undercompilation during incremental compilation after a
ctor change, like adding a parameter.

There is an existing scripted test, but coincidentally the test class
does NOT include packages, so the test ends up passing.

## Solution
1. This PR reproduces the issue by adding package name to the test.
2. This also fixes the problem by changing the `zincMangedName` to
`sym.owner.name ++ ";init;"`.

## Note about zincMangedName
`zincMangedName` in general is a good idea, which adds the class name
into otherwise common name `<init>`.
By making the name more unique it prevents overcompilation when one of
the ctors changes in a file.

compiler/src/dotty/tools/dotc/sbt/package.scala

@@ -4,17 +4,23 @@ import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.Symbols.Symbol
 import dotty.tools.dotc.core.NameOps.stripModuleClassSuffix
 import dotty.tools.dotc.core.Names.Name
+import dotty.tools.dotc.core.Names.termName
 
 inline val TermNameHash = 1987 // 300th prime
 inline val TypeNameHash = 1993 // 301st prime
 inline val InlineParamHash = 1997 // 302nd prime
 
 extension (sym: Symbol)
 
-  def constructorName(using Context) =
-    sym.owner.fullName ++ ";init;"
-
-  /** Mangle a JVM symbol name in a format better suited for internal uses by sbt. */
-  def zincMangledName(using Context): Name =
-    if (sym.isConstructor) constructorName
-    else sym.name.stripModuleClassSuffix
+  /** Mangle a JVM symbol name in a format better suited for internal uses by sbt.
+   *  WARNING: output must not be written to TASTy, as it is not a valid TASTy name.
+   */
+  private[sbt] def zincMangledName(using Context): Name =
+    if sym.isConstructor then
+      // TODO: ideally we should avoid unnecessarily caching these Zinc specific
+      // names in the global chars array. But we would need to restructure
+      // ExtractDependencies caches to avoid expensive `toString` on
+      // each member reference.
+      termName(sym.owner.fullName.mangledString.replace(".", ";").nn ++ ";init;")
+    else
+      sym.name.stripModuleClassSuffix

sbt-bridge/test/xsbt/ExtractUsedNamesSpecification.scala

@@ -306,7 +306,7 @@ class ExtractUsedNamesSpecification {
     // All classes extend Object
     "Object",
     // All classes have a default constructor called <init>
-    "java.lang.Object;init;",
+    "java;lang;Object;init;",
     // the return type of the default constructor is Unit
     "Unit"
   )

sbt-test/source-dependencies/constructors/A.scala

@@ -1 +1,3 @@
+package example
+
 class A(a: Int)

sbt-test/source-dependencies/constructors/B.scala

@@ -1 +1,3 @@
+package example
+
 class B { val y = new A(2) }

sbt-test/source-dependencies/constructors/changes/A2.scala

@@ -1 +1,3 @@
+package example
+
 class A(a: String)

sbt-test/source-dependencies/constructors/changes/B2.scala

@@ -1 +1,3 @@
+package example
+
 class B { val y = new A("a") }