4001 excess recompilation

[tswelsh]

Fixes #4001. I have assumed we always want to check internal library files for changes, but @mihaimaruseac will probably know more about this than me? Sorry if I'm stepping on your toes by the way!

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[mihaimaruseac]

Nice. I forgot to add the isCInternalLib in #3955 but I'm doing it in #3982 (https://github.com/commercialhaskell/stack/pull/3982/files#diff-726a384682e68eaa6ea53d0f5825efc1).

I think the approach you've taken is good, but will look more carefully later today, if I get some time.

[mihaimaruseac]

I tested it against all the internal-libraries related bugs and it doesn't break any of the already solved ones. It solves #4001, so it's good 👍

[tswelsh]

Great! The travis failure is unrelated, it's just a lint failure in a module I haven't touched.