Strip _build/default when locating files

[MackieLoeffel]

This PR upstreams the heuristic to locate the right file for projects using dune from ProofGeneral/PG#477 (comment).

I don't really understand what exactly is going on in company-coq-library-path and this PR is just a naive adaptation of the code linked above. Please feel free to suggest better ways to write this code. I am also not sure if one should allow customization of which parts should be stripped (e.g. allow the user to give a regex that should be removed).

[Blaisorblade]

Do you still extend _CoqProject by hand to use this? Is this just about opening the original source instead of dune’s copy in jump-to-definition?

[MackieLoeffel]

This just addresses the second point (jumping to the original source instead of dune's copy in jump-to-definition). You still need to have a _CoqProject file with the right paths.

[aa755]

this works when I jump to definition for a definition/inductive etc. However, it doesnt work when I jump to definition on a line of the form "Require Import x.y.z". It seems the handling of that doesnt even touch the codepath changed in this MR.

[Blaisorblade]

Thanks for fixing this @aa755 in master...aa755:company-coq:strip-build-dir, would you be okay submitting that as an MR?

@cpitclaudel Is there anyway we can help progress on this MR?