Root of Java select must be class or rooted package

[som-snytt]

Fixes #21333

Edge case in Java interop.

[som-snytt]

The edge case is imports. scala/bug#7675

[som-snytt]

-Ycheck apparently complains that Java class java in the empty package duplicates root package java.

I'm unable to reproduce the C.I. failure, which is only on Windows. Is that because the option is only specified for Windows? It reports -Ycheck:all, instead of specifying phase names.

java.lang.AssertionError: assertion failed: class defined twice module class java 6 2887
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
Error while emitting tests\pos\t11788b\Bar.scala
	at dotty.tools.dotc.transform.TreeChecker.testDuplicate(TreeChecker.scala:55)
assertion failed: class defined twice module class java 6 2887
	at dotty.tools.dotc.transform.TreeChecker.transformSym(TreeChecker.scala:78)
	at dotty.tools.dotc.core.DenotTransformers$SymTransformer.transform(DenotTransformers.scala:72)

Hopefully just putting the test code into named packages will solve the problem (which I can't reproduce).