Detect duplicate non-cyclical project imports

[philderbeast]

A follow on from #9578. Uses an IORef to better report on duplicates (including cycles) and to detect duplicates that are not cycles. This was originally included in #9578 but punted at review, #9578 (comment).

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[grayjay]

@Mikolaj Do you know if there is someone else who could review this pull request? This followup change doesn't really affect the dependency solver, and I'm currently focusing on #9743.

[philderbeast]

@TeofilC I'm inviting your review as cabal could perhaps be enhanced to output a list of files imported from a project in the future for use by other tools, input-output-hk/haskell.nix#2144 (comment).

[Mikolaj]

@alt-romes: could you maybe have a look?

[Kleidukos]

Looks like you may need to rebase on master so that the changes specific to #9578 don't appear anymore? At least the changelog entry for it is extraneous here. :)

[philderbeast]

Thanks @Kleidukos for pointing out the stray changelog entry. Rebasing didn't clear it so I deleted it and then squashed related commits.