Don't recompile in presence of sublibraries

[mihaimaruseac]

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!

---

Tested first with the internal-libraries package mentioned in #3899 (comment) and made Stack not ignore the internal library in this case.

Furthermore, tested against my own hakyll-based repo, to reproduce #3899.

[lierdakil]

@mgsloan, can this be merged/released sooner rather than later? Full rebuilds on every action are a huge pain.

[rcook]

Agreed. This is killing my productivity.

[mihaimaruseac]

If there's any need for more work on this, I'd be happy to do it.

[snoyberg]

Sorry for the delay on reviewing this. Two comments:

1. It would be ideal if you could add an integration test to ensure this works and that the bug doesn't recur in the future. (See: https://github.com/commercialhaskell/stack/tree/d0bf58a2a663bfb81e67a6af5e752387b22d4a2e/test/integration)
2. Would it be possible to do away with the usage of fromJust? We generally try to skip partial functions in the codebase

Thank you for this PR!

[mihaimaruseac]

Sure. I'll do both this week.

[mihaimaruseac]

And done, was easier than expected to add the test.

[snoyberg]

Looks great, thanks! @borsboom I think this is worth including in the 1.7 release.

[borsboom]

I've cherry-picked into the v1.7.0 (next release) branch.

[Porges]

@borsboom can we get a 1.7.0.2 build? I'm in so much pain 😁

[borsboom]

@Porges https://github.com/commercialhaskell/stack/releases/tag/v1.7.0.3

[Porges]

@borsboom Thanks! 🙂

Update: confirmed that this fixes the rebuilding.