Reproduce error from scala-exercises/sbt-exercise

[jisantuc]

This PR includes a reproduction of the error encountered in scala-exercises/sbt-exercise#117, but with way less going on. Moreover, it reproduces the error _without_dynamic class loading, and without the special Global subclass.

The two reproductions are:

• in SourceExtractSpec, initialize a CompilerJava class whose compile method similarly defers to a Compiler case class. It doesn't do anything interesting here though -- it just attempts to initialize a vanilla Global.Run() then return an empty array. This results in:

[info]   No RuntimeVisibleAnnotations in classfile with ScalaSignature attribute: package object collection
[info]      while compiling: <no file>
[info]         during phase: globalPhase=<no phase>, enteringPhase=<some phase>
[info]      library version: version 2.12.13
...

• in sbt-source-extract/simple/test, I attempt to call the task created by this plugin, that does the dynamic classloading song and dance, and get the same error

This is exciting because it means that what's wrong has nothing to do with sbt plugins or dynamic classloading, neither of which is terra firma for me.

CI will only run the first test, but if you clone and check out this branch you can run compiler / test and scripted separately to see that they both reproduce the error.

Closes #2

Closes #3