build a rule to generate the intra-target source deps

[johnynek]

Zinc:
https://github.com/typesafehub/zinc

could analyze sources and output the dependency graph of the files in a target. This could be helpful to output a set of targets that are smaller so the bazel's built in incrementalism is more effective.

so, you could have something like this:

scala_file_dependencies(name = "foo", target = "mylibrary")

and this could generate a new BUILD as output that has each file as a target with the correct dependencies.

This seems suboptimal for usability, but given bazel's model of compilation as a pure function from source to output artifact, this might part of a practical solution for making faster builds.

[johnynek]

this was done in #892