Remove unused, forgotten parameter from _scanvspec.
Adds some comments to _scanvspecs.
Bugfix for RT96321 reported by Volker Schatz.

When a META.yml specifies a dependency on one module without specifying
a version, that missing version retains the value of undef. Reported by
Volker Schatz, regarding Text-LevenshteinXS's dependency on Test.

See: http://cpansearch.perl.org/src/JGOLDBERG/Text-LevenshteinXS-0.03/META.yml