latex backend: full path in heading labels

[Octachron]

This PR fixes the label used for headings in the latex backend.

Previously, the latex backend used only the heading label, for instance,

(** {1:s_name Title } *)

generated

\subsection{Title\label{s_name}}

whereas the references used the full path

\hyperref[Module_name-s_name]{Title[p\pageref*{Module_name-s-name}]}

This PR fixes this discrepancy (observed on the odoc version of the OCaml manual) by using the full path in the heading translation.

[Octachron]

There seems to be multiple changes required to make the reference manual works with odoc 3, I am planning to switch this PR to non-draft mode once I have compiled all the changes.