This allows .flake8 files to override one another, and fixes a pretty bad known
bug with our flake8 implementation. For example, say we have a .flake8 file at:
/foo/.flake8

Before this patch, if we ran |mach lint foo/bar|, the configuration defined in
that .flake8 file wouldn't get picked up. It would only work if running the
specific directory that contains it, e.g |mach lint foo|.

This change additionally allows multiple .flake8 files to be used. So if
there's one defined at both:
/.flake8
/foo/.flake8

Then running |mach lint foo/bar| will first apply the root .flake8, then the
one under /foo (overriding earlier configuration).

This bug still doesn't make flake8 configuration perfect though. Any directory
containing a .flake8 file still needs to be explicitly listed in the "include"
section of /tools/lint/flake8.yml. Otherwise in the example above, if running
|mach lint /|, it wouldn't be able to find /foo/.flake8. This is a hard problem
and is likely best solved by fixing flake8's upstream configuration handling.

Unfortunately this means we still can't switch from a whitelist to a blacklist.

MozReview-Commit-ID: 3DZAi1QHYYo

--HG--
extra : rebase_source : 51298c5847f6c2792581d9b312c87b70fa716ee1