Should warn about overlapping "hs-source-dirs"

[mgsloan]

The logic should be that if a package component declares a dependency on a library or sub-library in the package, then hs-source-dirs should not overlap. This is because imports that occur within the executable / test-suite / benchmark / sub-library etc will instead recompile the code, which is a waste.

It also potentially has other negative effects - #3109 (comment)

[Asalle]

I would like to fix this.
Do I understand it right: I just have to look through all the dependencies of hs-source-dirs and other libraries and delete duplicates?

[Asalle]

it's not clear to me, what is a package component and how to declare a dependency in the package. Can you have multiple package.yaml files?

[mgsloan]

@Asalle Sounds great, please have at it! A package component is either a library (the usual thing that a package wraps), or an executable, test, or benchmark. See the executables / tests / benchmarks sections of package.yaml.

Note that it shouldn't warn if 2 executables use the same hs-source-dirs, this is often fine. The logic should be:

If the component depends on the library defined in the current package, then check each of the hs-source-dirs in the component and see if there is overlap with the library. If there is overlap, warn about it.