Fix undercompilation upon ctor change

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

[Cherry-picked 157ed43]

Author: eed3si9n

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

@@ -12,7 +12,7 @@ inline val InlineParamHash = 1997 // 302nd prime
 extension (sym: Symbol)
 
   def constructorName(using Context) =
-    sym.owner.fullName ++ ";init;"
+    sym.owner.name ++ ";init;"
 
   /** Mangle a JVM symbol name in a format better suited for internal uses by sbt. */
   def zincMangledName(using Context): Name =

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;",
+    "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") }