build a rule to generate the intra-target source deps

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.

Start by investigating Zinc's source-analysis capabilities and how the proposed scala_file_dependencies rule would fit Bazel's target model. Define the input target, dependency graph, and generated BUILD output before attempting implementation; done would mean a working rule that produces per-file targets with correct intra-target dependencies.

Written by the indexing model from the issue text.