Fix local+noindex repos on Windows

[jasagredo]

This PR implements the changes described in haskell#10647 (comment). Commit messages should describe each change and are self-contained.

Checklist:

• 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!) Tests now pass

[jasagredo]

cc @9999years