Be more permissive when classifying links as kind LINK_URI

[JorjMcKie]

Discussed in #3298

When extracting links, be more permissive when determining the "kind" of a link as LINK_URI.
We should accept all external links containing a colon, ":" in their URI string as LINK_URI if they do not explicitly start with "file://".

[julian-smith-artifex-com]

Fixed in 1.24.1.