Add support for `package_url` field

[tdruez]

See for example https://github.com/nexB/aboutcode-toolkit/blob/develop/thirdparty/saneyaml-0.1-py2.py3-none-any.whl.ABOUT#L17

Running aboutcode-toolkit version 3.4.0.pre1
Checking ABOUT files...
INFO: saneyaml-0.1-py2.py3-none-any.whl.ABOUT: Field package_url is a custom field.

See also https://github.com/package-url/packageurl-python

[pombredanne]

@tdruez this make sense... though there is eventually some decision to make in terms of component (which is what name and version document in the current spec) and package (which is what a package_url documents, including name and version).

We could say that name and version apply to the package.
And add new component_name and component_version

Then a package_url just provides extra data? (type, qualifier, subpath?)
Instead allow to state each Package URL attribute as its own field?

[chinyeungli]

any decision?

[chinyeungli]

This is the purl spec: https://github.com/package-url/purl-spec