Add web interface where someone could point to a URL of a package to collect and scan

_Originally posted by @armijnhemel in https://github.com/nexB/purldb/issues/75#issuecomment-1519895893_

> Possibly related, but what also would be cool if this would be a web interface where someone could point to a URL of a package and indicate that it is interesting to be harvested (perhaps with a little bit of extra data for triage to avoid spam/malware being indexed?)

> There are many projects which aren't on GitHub or other well known forges (but less well known forges, or completely independent hosting) or which are only released as tarballs on a website.

> I would not expect a user to be able to create a valid PURL so making it a low barrier entry thing would be nice. I am not sure if I would make it completely automated (spam, malware, duplications, etc.).