Refactor before import warning

[philderbeast]

Split from #10933, the refactoring and the sorting changes without the duplicate import by alternate paths detection, as requested by @ulysses4ever.

• Add a new module cabal-install/src/Distribution/Client/ProjectConfig/Import.hs and move some stuff there from cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs that was only ever used in cabal-install.

For messages, I added some doctests and ended up with two ways of comparing ProjectConfigPath.

-- | A comparison that puts projects first, URLs last and sorts the other paths
-- lexically.
compareLexicographically :: ProjectConfigPath -> ProjectConfigPath -> Ordering

-- | A comparison that puts projects first, URLs last and sorts the other paths
-- by putting longer paths after shorter ones as measured by the number of path
-- segments. If still equal, then sorting is lexical.
compareSegmentally:: ProjectConfigPath -> ProjectConfigPath -> Ordering

I'll squash commits before applying the merge label if this pull request is approved.

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[ulysses4ever]

Thanks a lot for splitting it --- it helped reviewing a lot! A couple nitpicks inline.

[ulysses4ever]

One little nitpick: I can't quite parse the PR title "Refactor/before import warning". Could you turn into something more readable?

[philderbeast]

Sorry about that @ulysses4ever. That was the branch name. I've removed the needless "/" separator.

[Mikolaj]

Thank you.