Set 'path' and add fallback for 'includeexpr'

[mateoxh]

Until the server is started the command gf and insert-mode completion for
included files don't work, so I changed &path to point to Rocq's libraries
directories and added a fallback to &includeexpr when the query to Rocq
fails.

One small caveat is that for gf the filename is first searched in &path and
if it can't be found it is modified with &includeexpr and another search is
done, so (in theory) it may be possible that the file opened by gf is not the
same one found by Rocq (&includeexpr is always used on the files found with
&include).
I don't think it's a big issue but remapping gf is a simple solution.

[mateoxh]

Is that the correct find_coq function? It's not coqidetop but it accepts the same flag and it works.
Also, here is the only place you check for the user defined variables, not sure if that call can be replaced with the new function in coqtail/util.vim.

[whonore]

Thanks, I think this looks good. I'll double check that everything works ok tomorrow.