Quick support for includes from local Antora hierarchy

[alaindresse]

We use antora to write documentation on libs and (react-native) apps in a large nx monorepo.
This setup generates quite a few symlinks, and in consequence the vscode plugin takes ages to update, to the point of being completely unusable.

The revamping of findFiles in vscode does help quite a lot (I have a working version for codeInsiders using findFiles2), but there is no clear ETA for when the revamping will be complete.

In the meantime, I realized that >95% of our needs could be met by managing includes (and maybe xrefs) of the type family$resource.ext or possibly module:family$resource.ext for a module that is in the same antora hierarchy as the edited file.

I will create a PR implementing this.