Cabal and GHC environment file related errors

Inspired by [this stackoverflow question](https://stackoverflow.com/q/73329310).

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.