Find Antora cross-references project-wide

[bbatsov]

Phase 2c of the Antora/xref work, completing the cross-file story.

In an Antora component, M-? (xref-find-references) on an id now searches the whole component rather than just the current buffer. So you see every cross-page reference - xref:this/page.adoc#id[] from other pages, including module:-qualified forms - alongside the same-page <<id>> / xref:id[] usages. Outside a component it stays in-buffer as before.

It's built on Emacs' own xref-matches-in-directory (grep). The match regexp distinguishes by page+fragment, so a same-id-but-different-page reference isn't a false positive, and prefix collisions (foo vs foobar) are excluded.

Notably I tested this against both BSD grep (my machine) and GNU grep 3.12 (the CI/Linux flavor) - an earlier version used a shy group that BSD grep tolerated but GNU grep rejects, which would have broken M-? on CI; the committed regexp is a flat alternation that both accept.

That wraps up Phase 2 (Antora xref: following, completion, references). Next: paste-image-from-clipboard.

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