Fixes #10636

This PR fixes the linked issue by making the stylesheet path absolute before passing it to Haddock, and also passing the stylesheet to the index generator as well.

QA Notes

Calling cabal haddock-project --css=<css-file> on a multi-package project where every package has its own subdirectory (and not have their source files at the project root) should produce documentation such that:

• The index (i.e. ./haddocks/index.html) is styled by the provided CSS file
• Each package has their docs (i.e. ./haddocs/<package>/<module>.html) styled by the provided CSS file

This reproducer of the original issue can be used for QA purposes.

Template Α: This PR modifies behaviour or interface

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