clean does not seem to work with java projects

[johnynek]

steps

1. Make a minimal subproject in a multiproject build that has a few files of only java.
2. compile the code: mysubproject/compile
3. view that the class files are generated
4. clean: mysubproject/clean
5. verify that the classes are gone
6. compile the code again and see that nothing happens.

problem

It seems that some in-memory state in sbt is failing to be synced with the disk state.

Workaround: exit sbt after the clean and restart and it works. Again this suggests it is memory state, not disk state that is incorrect.

expectation

javac should be invoked again on the sources.

notes

you can see an example of this in this repo: johnynek/bosatsu#974

check it out at johnynek/bosatsu@c5a623d and
try this set of steps:

1. truffle/compile
2. truffle/test (should pass)
3. truffle/clean
4. truffle/test (now it fails, and compile won't fix it. exit sbt and restart and it should work again).