Support path mapping

[jadenPete]

This issue is part of a broader proposal to merge some of the features in lucidsoftware/rules_scala into this ruleset.

The tracking issue for this effort

The relevant proposal section.

Path mapping should prevent issues with targets from being built multiple times in multi-toolchain repositories. These issues are discussed in more detail in the proposal, but the gist is that if a target is depended on under different configurations, it may be built multiple times, even if the difference in configuration doesn't affect how the target is built. This is because what toolchain is used to build a Scala target depends on Bazel builds targets under different output directories, depending on the configuration. Path mapping attempts to solve this by "[rewriting] paths in action command lines with the aim of making them more likely to be disk or remote cache hits", and it's solved this issue for us over at lucidsoftware/rules_scala.