This forward ports Mixbus' 4eca3dea47 without additional code to correct
for old (Ardour2) URIs.
A2, used path2uri() which accumulated and left duplicate slashes in place,
eg. "file:////". Mixbus3 auto-corrected this on load in
AudioLibrary::search_members_and().
However sfdb libraries are not shared between major versions, so this
code does no need to be forward ported.