Add support for comma-separation to "loose" classifier

[suve]

An unfortunate, but somewhat common pattern seen in licence strings is the use of a comma to list multiple licences, like so:

Licence-A, Licence-B, Licence-C

It would be good to add some support for this form to the loose classifier.

Things to consider:

• Just because a string contains a comma, it doesn't mean the comma must be a separator. For example, Fedora used the Redistributable, no modification permitted string for a long time to describe firmware blobs.
• It is not possible to determine if A, B is supposed to mean A or B or A and B. Since the former interpretation increases the risk of false-negatives (non-free software being listed as free), the latter interpretation should be used.
• When presenting the licensing breakdown to the user (via the --explain option), the program should try to remain as close as possible to the input string and use a comma to separate the individual licences.