Fix #525: Spuriously duplicate options

[jchyb]

This PR fixes the issue and adds a unit test based on the original issue snippet. It is worth noting that I decided to fix only the spuriously duplicate options - users still can duplicate them on will f.e. by providing multiple of the same using directives in different files or by adding an already defined option via cli. On that occasion they will most likely still receive the same warning.

I did not want to limit users in defining duplicate options as that would require a more strict control of a scalac api, f.e. perhaps one day scalac will have an option that will be able to be defined multiple times (like our verbose) and then our hypothetical assumption would become incorrect.

[alexarchambault]

Just tried something slightly different in the commits I just pushed, as the CI errors looked legit… Feel free to revert them, if ever my approach doesn't pan out well.

[alexarchambault]

The idea of my approach is that CrossSources basically splits its buildOptions: Seq[HasBuildRequirements[BuildOptions]] field in two: some options go in def sharedOptions, the others go into the ScopedSources returned by def scopedSources. My changes make sure options go only in one of those, not both (as was the case before it seems).

[jchyb]

Thank you for the fixes, I must have messed something up with the scopes before. I'll be more careful next time

[alexarchambault]

Thank you for the fixes, I must have messed something up with the scopes before. I'll be more careful next time

No problem, that's a super hairy area of the code base. As the commit history shows, I definitely didn't address that in the first try 😅