When a requirement is requested multiple times, some via a direct URL
("req @ URL") and some not but with extras ("req[extra] VERSION"), the
resolver previous could not correctly find "req[extra]" if "req" is
available in an index.

This additional logic makes the resolver, when encountering a
requirement with identifier "req[extra]", to also look for explicit
candidates listed under "req", and add them as found matches for
"req[extra]".

Also moves the incompatibility candidate calculation to closer to their
usages.