path for hcp coverage report

`stack` has a feature `stack path --local-hpc-root` which retrieves the path to the coverage report generated by `hpc`.

Would it be possible to have a similar feature in `cabal`?

It would be nice so that it becomes easy to call something `xdg-open $(cabal path --local-hpc-root)/hpc_index.html`

Start by locating Cabal's existing `path` command and the handling of local coverage output, then compare its behavior with Stack's `path --local-hpc-root`. Implement the analogous option so it returns the directory containing `hpc_index.html`, and verify that the suggested `xdg-open $(cabal path --local-hpc-root)/hpc_index.html` workflow resolves the expected path.

Written by the indexing model from the issue text.