Fix reading of scalac plugin info from config.

The relevant keys are now:

[compile.scala]
plugins=['foo', 'bar']
plugin-args= {
  'foo': ['bar']
}

Testing Done:
Manually verified that the args are picked up.

Reviewed at https://rbcommons.com/s/twitter/r/1217/
Sync of https://github.com/pantsbuild/pants/commit/64cf6f995ea2e49af4793b1058abcecbcbaadad9