Split the functions that create URLs and fetch into two separate functions

> We are going to have discrete functions each with a router that return a very specific URL or small piece of data like today. But here the functions that create URLs and fetch should be split in two so that we can have URL-only functions as explained in 2. that do not fetch anything.
>
> Functions that only transform a PURL in a URL should be in one place (likely packageurl-python)
>
> Then anything that fetches remote data should be of two kinds
>
> One may return raw data from a JSON or XML API
> One may return a ScanCode Package object converted from this raw data
> Some basic function that only return versions may just return lists of PURLs alright
>
> We need to account to for repository_url and download_url qualifiers

see https://github.com/nexB/fetchcode/pull/93#issuecomment-1828290140