New docbook-xsl (1.79.2) breaks existing XSL files

[Leonardo Brondani Schenkel]

Ticket: https://trac.macports.org/ticket/55255

DocBook XSLs changed their URIs from `sourceforge.net` to `github.com`, 
which results in existing XML files using the former URIs not able to 
find the local copy installed by the port. This will trigger a download 
from the URI or fail if net access is disabled.

This could break many existing ports and user XML files. Neomutt for 
example can no longer be built: https://trac.macports.org/ticket/55253

I'm writing to the list because this change in upstream might have far 
reaching implications, and this port has no official maintainer, so I 
want to bring as much visibility to the issue as possible and discuss a 
solution.

Thanks,
// Leonardo.