WIP Add support for Fossil source repositories

[dxtr]

I'd like to be able to use my repositories stored in fossil as source repositories in cabal.

I have written tests for it but all the tests in cabal-install/tests/UnitTests/Distribution/Client/VCS.hs seem to be ignored for some reason.

• [ ] 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!)