Adds config option repositoryUrls which can be used by pip to define an alternate host to pypi.

Closes #2181

Renames "date" and "time" values from datasources to instead be "releaseTimestamp".

Closes #2085

Adds a wrapper to all datasources to provide a generic purl-based interface.

Adds pypi datasource, compatible with existing npm and github ones. Performs no validation/filtering of versions.