Override version equality constraints

If we were able to override version equality constraints then we would not have to download a stackage resolver's `cabal.config` file locally and comment out conflicting versions. This would work much the way stack allows its version overrides.

> In 3.8 we will have conditionals and includes (including remote) in
> cabal project files. So it will cover many use cases. However, we will
> not yet have the ability to override constraints in included files. So
> if you include a given lts, you will not be able to bump the version
> of anything in the lts, outside of downloading and modifying its
> freeze file, then depending on the modified freeze file.

_Originally posted by @gbaz in https://github.com/haskell/cabal/issues/7556#issuecomment-1120433903_