Provide fallback link text for self xref

If a page refers to itself using only a relative path, and that xref doesn't specify link text, the link text appears as an empty set of square brackets.

For example:

xref:current-page.adoc[]

Appears as:

[]

This is related to https://github.com/asciidoctor/asciidoctor/issues/4032. This can be resolved by detecting the signature of a self xref and setting the refid to the relative src path of the current page.

This issue is resolved when the fallback link text for a self xref resolves just like an inbound xref to that page.