Fail early & more clearly on shaded-broken classfiles

[dwijnand]

The shading utility uses for the coursier or coursierapi classes has
removed the RuntimeVisibleAnnotation attribute after the ScalaSignature
attribute. Parsing such a classfile caused the Scala 3 compiler to
parse the file as a Java classfile. It seems there something about how
we deal with packages and package objects (which don't exist in Java)
when we are incorrectly sent down this code path. The Scala 2 compiler
already correctly caught and failed on this early, with this detail, so
look to do the same here.

Automating the test for this isn't easy. Because relying on an external
dependency would be a hazard for CI, long term. Perhaps we could try to
recreate a classfile with a similar bytecode problem, but that would be
quite involved as well. I guess we could commit some or all of the
classfiles from the original, but it's generally advised not to commit
binaries to the source repo.

So instead I'm leaving instructions as to how I manually tested this, in
preparing this change. I used coursier's cs binary to download the
jars and produce a classpath string:

 cs fetch -p org.scalameta:scalafmt-dynamic_2.13:3.8.1

Then I invoked scalac in the sbt shell, passing that classpath string:

scalac -classpath $classpathAbove i20405.scala
scalac -classpath $classpathAbove i20555.scala

Using the two minisations in the 20405 and 20555 issues.

Fixes #20405
Fixes #20555