Avoid forcing whole package when using -experimental

In #19807, the behavior of `-experimental`
was changed to mark all top-level definitions as experimental. To do so, the
implementation traverses the whole package and checks every symbol to see if it
should be transformed or not. The problem was that the first check we do is
`sym.isExperimental` which ends up forcing the symbol.

Besides being a performance issue, this could also lead to a crash if the
current package is the empty package, because we could end up forcing the magic
`module-info.class` that Java modules place there. For some reason, this appear
to only happen when building with sbt, hence the additional scripted test.

This PR fixes this issue by reordering the checks (and adding a preliminary
`isDefinedInCurrentRun` check for good measure). We should also investigate
whether we can avoid creating a symbol for `module-info.class`, but this PR is
intentionally minimal so we can backport it to 3.5.0-RC2 without risks.

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -806,10 +806,11 @@ object Checking {
   def checkAndAdaptExperimentalImports(trees: List[Tree])(using Context): Unit =
     def nonExperimentalTopLevelDefs(pack: Symbol): Iterator[Symbol] =
       def isNonExperimentalTopLevelDefinition(sym: Symbol) =
-        !sym.isExperimental
+        sym.isDefinedInCurrentRun
         && sym.source == ctx.compilationUnit.source
         && !sym.isConstructor // not constructor of package object
         && !sym.is(Package) && !sym.name.isPackageObjectName
+        && !sym.isExperimental
 
       pack.info.decls.toList.iterator.flatMap: sym =>
         if sym.isClass && (sym.is(Package) || sym.isPackageObject) then

sbt-test/java-compat/moduleInfo/A.scala

@@ -0,0 +1,2 @@
+// Previously, we crashed trying to parse module-info.class in the empty package.
+class A

sbt-test/java-compat/moduleInfo/build.sbt

@@ -0,0 +1,5 @@
+scalaVersion := sys.props("plugin.scalaVersion")
+
+scalacOptions ++= Seq(
+  "-experimental"
+)

sbt-test/java-compat/moduleInfo/test

@@ -0,0 +1 @@
+> compile