Allow ghc to invoke javac

[facundominguez]

inline-java invokes javac in a GHC Core plugin in order to compile generated java stubs. The bytecode is then embedded in the object file of the module.

Bazel, however, requires all inputs to be declared to a rule, or the inputs won't be visible to the compiler. In particular, we would need to somehow say that javac is an input to the rule, or the GHC plugin won't be able to invoke it.

I believe that, currently, there is no way to specify programs as inputs to the haskell build rules, so this issue would be about implementing some way to allow it.

sparkle and jvm-batching used to build with rules_haskell at commit 23d8b0f, probably because of a bug in rules_haskell. But this is no longer the case (fcef649). If we can't find a good way to declare javac an input to the build rules, it may worth investigating how it did work before and attempting a hack, but I'd prefer not to start like that.

[facundominguez]

After some discussion with @mrkkrp we are heading for adding a new extra_binaries attribute to the haskell_toolchain rule that gets a list of labels. Here we should list javac, which then will be included in the folder visible-binaries.