Refactor rules into configurable phases

[andyscott]

This refactors the core rules into a series of more isolated and configurable phases that can be run together as a pipeline. Additionally, it enables plugins to insert new phases into the pipeline for a particular rule.

This should enable the following future work:

• Remove the "scalac" rules and instead have a non-zinc compile phase that can be used for bootstrapping the zinc phase.
• Scalac plugins that add providers to the output. In particular, I'd like to add a SemanticDB compiler plugin that inserts a SemanticDBInfo provider in the final output.
• ScalaJS/ScalaNative pipelines that share a lot of the same infrastructure as the existing pipelines.

[andyscott]

(I think there's one lingering issue where the SingleJar is combining files in a different order than we originally expected, but only in CI.)

[JaredNeil]

Having a big comment block before each of these phases makes me think they should probably each be in their own file.

[andyscott]

I think so too. I have a few more changes in other PRs to iterate on. These will influence the phase names and interfaces (in the global g struct), so I'll want to do wrap them up before splitting phases to separate files. I'd also like to organize the associated Scala source files along with the phases: the deps checker Scala can sit in the same area as the deps checker Starlark code.

[JaredNeil]

Looks good to me.

[borkaehw]

Looks good to me. One question though, are we planning on doing similar thing to proto and format?

[andyscott]

I don't think the proto and format rules share the same phases. If they do, then we could perhaps do something similar down the road.

[andyscott]

I am going to go ahead and merge this one, as it heavily impacts future work.