JavaCompiler: need better way to organise Java dependencies within a package

[lhoward]

If I want target B to depend on a Java class compiled by JavaCompilerPlugin in target A, then I need to explicitly set CLASSPATH in the environment to point to the build artefacts directory. [1] This should be done by the plugin passing the appropriate argument to javac.

[1] which results in other errors but that will be the subject of a separate issue