Normalize more paths in coqdep to avoid spurious warnings

[rlepigre]

This fixes warnings such as the following:

Warning: ltac_plugin.cmxs already found in [...]/_opam/lib/coq-core/plugins/ltac (discarding [...]/_opam/lib/coq/../coq-core/plugins/ltac/ltac_plugin.cmxs)
         [multiple-matching-files,filesystem,default]

[SkySkimmer]

Seems fine to me but I'm not a coqdep expert.
@ejgallego @Alizter I think you are more up to date on coqdep, any objections?

[Alizter]

We need to make sure this works correctly on windows. Are we testing Coq platform for windows in the CI?

[ejgallego]

Thanks @rlepigre , I was actually looking into this the other day; the "right" fix would be to have Coq stopping to set coq/../coq-core as a path. However this change requires a bit of deep surgery on the config code, so I didn't have the cycles to finish it.

[Basically you want to move from the current setup where coqlib is the "root" for a Coq install to a true prefix root, where packages are then located]

Happy to have this in Coq meanwhile.

Regarding testing, I think using ppx_inline_test or use unit tests to check the implementation in all architectures would be the best option.

[ejgallego]

Seems fine to me but I'm not a coqdep expert. @ejgallego @Alizter I think you are more up to date on coqdep, any objections?

Patch is an improvement over the current situation.

[SkySkimmer]

@coqbot run full ci

[SkySkimmer]

@coqbot merge now