Enable calling d2d when collecting/indexing a package or on demand via the API

This is a follow up from https://github.com/nexB/purldb/issues/373

I would like to have a PurlDB REST API endpoint so that we can trigger a d2d from either one of these (design needed):

- a pair of from and to PURLs to run d2d on
- a package set for one PURL and run pair-wise d2d for each package in the set
- a single PURL for some binary that is collected and where we found the sources as part of
- https://github.com/nexB/purldb/issues/418
and then we run d2d between these two

This could be an on-demand analysis or part of the standard indexing and we could store the analysis discrepancy results produced by:
- https://github.com/nexB/scancode.io/issues/1148
in each package extra data

Some related issues wrt. to the package models are:

- [ ] https://github.com/nexB/purldb/issues/186
- [ ] https://github.com/nexB/purldb/issues/387