Backport #10728: Fix file+noindex URI usage on Windows

[mergify]

This PR fixes the parsing of URIs for file+noindex repositories when using Windows paths. As suggested by @phadej in #10703 we now use (and specify in the docs) //./C:/... paths on Windows.

QA

In Windows, one can now specify //./ paths in file+noindex repositories. To check, create a simple package, then cabal sdist, move the tar.gz to some directory and in a different project declare the following stanza:

repository my-local-repo
  url: file+noindex:////./C:/path/to/repo

It might still fail because of #9891

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Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

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• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• 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!) fixing other tests failures seems like a good enough test for me

  ---

  This is an automatic backport of pull request Fix file+noindex URI usage on Windows #10728 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   Cabal-syntax/src/Distribution/Utils/Path.hs
	modified:   cabal-install/src/Distribution/Client/Config.hs
	modified:   cabal-install/src/Distribution/Client/GlobalFlags.hs
	modified:   cabal-install/src/Distribution/Client/IndexUtils.hs
	modified:   cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
	modified:   cabal-install/tests/UnitTests/Distribution/Client/ProjectConfig.hs
	modified:   cabal-testsuite/src/Test/Cabal/Prelude.hs
	new file:   changelog.d/pr-10728
	modified:   doc/config.rst

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Makefile
	both modified:   cabal-testsuite/src/Test/Cabal/OutputNormalizer.hs

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