Bug 1412460 - Move build output processing code out of mach_commands.py; r=mshal

In general, we shouldn't have so much business logic in mach_commands.py
files.

I'd like to move more of the low-level "perform a build" logic out of
mach_commands.py to facilitate some future work. That code makes use of
this output processing code. So as the first step in moving the build
code, we move its dependencies.

As part of this, I also cleaned up some random unused imports around
lines that were touched.

No meaningful code changes were performed as part of the code move.

MozReview-Commit-ID: 96mGWUJ7oLb

--HG--
extra : rebase_source : 04c3865d5bcc44777749f3ab6a65e31bb1d13689