Revert "Clean up --only-dependencies logic in ConstructPlan"

[rjmk]

This reverts commit d1e6c48 and resolves #3476

The problem is caused specifically in the PSFiles -> Nothing branch of installPackages.

As the flag was no longer being passed through, packages were not being checked for being in the wanted map. This led to the local packages being built as dependencies

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.

I tested this by running stack --only-dependencies --dry-run on our stack project with multiple packages. Setting the call to resolveDepsAndInstall's treatAsDep value to True causes the problem and removing it removes the problem

[mgsloan]

Hmm, I guess I did not consider the logic fully when implementing that change. My bad! The logic there is still rather confusing, but if this fixes the current case, then seems reasonable.

Thanks for tracking it down!