Configure overwrites cabal.config with constraints

`configureAction` has access to the `LocalBuildInfo` value determined during
configuration. Exact version constraints for the dependencies can be
determined from it. All of those constraints are unconditionally written to
`cabal.config` overwriting the entire file.

Work that still needs completing:

 - Make the overwriting conditional.
 - Perhaps omit certain dependencies, such as `base`.
 - Replace only the `constraints:` section of `cabal.config`.