In order for the `include_only` linter rule to not have
any side effects, we need to explicitly add a trailing '/'
after every directory we want to include.
The main thing here is to make looping over lines be the inner
loop, instead of looping over rules. This keeps regexes in
cache, and it also avoids some O(N) checks.
This is a significant speedup for me, reducing time from 16s
to 11s.
As part of extracting this, we exempt the library from all custom
checks on itself. This is expedient, since a lot of our
custom checks are naive about whether things are in strings, and
it is also a pain to configure individual rules.
