Return empty purl fields in purl_to_lookups

[JonoYang]

purl_to_lookups in django/utils.py returns the purl fields that have values. It would be nice to have an option for it to return all purl fields, even if the field was not populated. This is helpful in the case where we have two packages, pkg:maven/net.minidev/accessors-smart@2.4.9 and pkg:maven/net.minidev/accessors-smart@2.4.9?classifier=sources. When I use purl_to_lookups on pkg:maven/net.minidev/accessors-smart@2.4.9, and then use that to get a Package from the database, it returns pkg:maven/net.minidev/accessors-smart@2.4.9 and pkg:maven/net.minidev/accessors-smart@2.4.9?classifier=sources, when I just want pkg:maven/net.minidev/accessors-smart@2.4.9

[tdruez]

Merged in #156

There's now the possibility to filter on exact match.