Add support to fetch purl

[TG1999]

fixes: aboutcode-org/scancode.io#1383

The API design would look something like this

>>> from fetchcode import fetch
>>> resp = fetch("pkg:pypi/[email protected]")

[tdruez]

See my comments. Also there's a lack of unit test for this code.

[tdruez]

Test coverage is too light, it does not cover properly the new code.
Also, the mocking is not ideal.

Lastly, that new PURL support should be documented, and probably displayed in the README

[tdruez]

Look good for merge.