SI-7008 @throws annotations are now populated in reflect

Runtime reflection in JavaMirrors previously forgot to fill in
@throws when importing Java reflection artifacts. Now this is fixed.

Note that generic exception types used in `throws` specifications will
be garbled (i.e. erased), because we don't use `getGenericExceptionTypes`
in favor of just `getExceptionTypes` to stay compatible with the behavior
of ClassfileParser. That's a bug, but a separate one and should be fixed
separately.

Also note that this commit updated javac-artifacts.jar, because we need
to test how reflection works with javac-produced classfiles. The sources
that were used to produce those classfiles can be found in the jar next
to the classfiles.
evicts javac-artifacts.jar

Apparently, the usual _1, _2, _3... naming scheme also works for java
files, which need to be compiled together with partests. This allows us
to get rid of javac-artifacts.jar.
pullrequest feedback

https://github.com/scala/scala/pull/2040
reflecting @throws defined in Scala code

As per Jason's comment: How are Scala classes containing @throws annots
treated? I can't figure out whether we pickle the annotation in addition
to adding the exception to the signature. If we do, might we end up with
duplicate annotations in runtime reflection? This warrants a test.
See the context of the discussion here: https://github.com/scala/scala/pull/2040/files#r2874769.

No, we won't end up with duplicates, because classes defined in Scala
are loaded in a different completer. But I'll add a test - you can never
have too many of those.
Merge pull request #2040 from scalamacros/ticket/7008

SI-7008 @throws annotations are now populated in reflect