Log if there are multiple local packages with the same ID in a project

[edmundnoble]

Fixes #11140 by logging a message of the form:

  unlines
        [ "cabal project has multiple sources for pkg-one-0.1:"
        , "  <some path>/pkg-one"
        , "  <some path>/pkg-two"
        , "the choice of source that will be used is undefined."
        ]

It works for remote source repos + other remote repos as well, though it doesn't try to be too smart about printing remote source repo information to avoid verbosity.

Note: this message is only ever printed if the entire package identifier is duplicated between local packages, which includes both name and version.

One thing I'm not sure about here is how many packages already do this. Unfortunately clc-stackage is seemingly irrelevant as a test case here because cabal.project is the real issue. I'm also not sure if cabal2nix or haskell.nix generates project environments where this sort of message would be triggered a lot.

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