Complete Antora xref targets and fragments

[bbatsov]

Phase 2b of the Antora/xref work, on top of #81.

Inside an xref: in an Antora component, completion now helps you author the reference:

• the target position offers the component's pages (e.g. topic/page.adoc), with pages in other modules prefixed module:, plus the in-buffer anchor/section ids (for same-page refs)
• after a #, it offers the target page's section ids and anchors, read from the resolved page
• a same-page xref:# completes against the current buffer

Dot-prefixed entries (editor lock files, hidden dirs) are filtered out, and outside an Antora component the xref: completion falls through to the existing in-buffer behavior. Verified against cider/doc.

Next: project-wide xref-find-references (Phase 2c), then paste-image.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)