Support deploy environments (deploy_env)

(subsumes #560, expands on bazelbuild/bazel#1402)

Bazel has an internal feature called `deploy_env` for java_binaries, which allows performing classpath subtractions for specified targets (see a comment by Ulf [here](https://github.com/bazelbuild/bazel/issues/1402#issuecomment-226440001)).

We tested this feature (by cherrypicking the commit to expose it in java_binary) and it is suitable for our needs for specifying different deployment targets having different dependencies (something we can't use `neverlink` for).

As other people seemed to find it useful as well, I would like to propose adding support for this for scala_binaries. The classpath subtraction can be done in skylark using the code we already wrote, to support deployment scenarios explained in #560.

WDYT?
cc @ittaiz

Start by reviewing the existing Skylark classpath-subtraction code and the deployment scenarios described in #560, then compare them with Bazel's internal deploy_env behavior for java_binaries. Done means scala_binaries support deployment targets with different dependencies without relying on neverlink; the issue does not name specific files or tests.

Written by the indexing model from the issue text.