Improving GHC environment file related errors

[noughtmare]

Inspired by this stackoverflow question,

When running commands such as cabal install --lib bytestring the result may depend on what is already in the default environment file. However, cabal will not really tell you that. It only says things like:

bytestring-0.10.12.1 (constraint from user target requires ==0.10.12.0)

And

bytestring-0.10.12.0 (constraint from user target requires ==0.11.3.1)

I think it would be good to show exactly where these constraints come from, e.g. with file names and line numbers.

Also I think it would be useful to have a general warning/info message about which environment files are used.

[noughtmare]

I guess this is related to #7820.