purls not generated for unknown types

[cpendery]

What happened:
No purl is generated from URL when the type is unknown by Syft

What you expected to happen:
I expected purls to be generated even when there is not a known purl type and placed under generic

How to reproduce it (as minimally and precisely as possible):
It's listed as a todo item in the code

syft/syft/pkg/url.go

Line 45 in 2f1aa33

// TODO: should this be a "generic" purl type instead?

Anything else we need to know?:

Environment:

• Output of syft version: 0.52.0