Do not report .ABOUT files as "about" PURL types

See this by @silverhook :
- https://github.com/package-url/purl-spec/issues/397

> **Clarify what is `pkg:about` PURL**

> I recently came across `pkg:about/sourceforge/rdf-filter@1.0alpha` and a few other such PURLs in a ScanCode.io scan.
>
> The further info in ScanCode.io’s webUI says:
>
> > Homepage URL
> >
> > https://rdf-filter.sourceforge.net/
> >
> > Download URL
> >
> > https://master.dl.sourceforge.net/project/rdf-filter/rdf-filter/RDF%20Filter%201.0alpha/rdffilter-1.0alpha.zip?viasf=1
>
> But I still can’t figure out what `pkg:about` is.
>
> Is this a missing entry in this spec, or a bug in ScanCode.io?