Add support for cabal-install `active-repositories` option

cabal-install-3.4 will support an active-repositories option in cabal.project which allows a repository to "override" packages of another package. For instance, one might write:

active-repositories: hackage.haskell.org, head.hackage:override

This requires that if head.hackage provides a version of a package, then the solver may not use any version provided by hackage.haskell.org.

We should include this option in the generated cabal.project.local snippet.

Edited Jul 15, 2020 by Ben Gamari