Create a pipeline to pre-populate the PurlDB

[pombredanne]

I would like to ensure that PurlDB has up-to-date packages for my projects.
For this I would like to have a pipeline that does the following:

• Run a package-only scan on a codebase as a new pipeline - Proactively scan and review all my packages scancode.io#815
• for each detected package, call the purldb to trigger the mining and indexing of these package versions, using the new endpoints in On-demand JAR processing #64
• same for each dependency. For deps that are not resolved, we could either get the subset of versions that match the dependency constraints (using "univers") or first get the latest version and later all the versions, starting from the latest to the oldest.

This could be either integrated in an other pipeline or be a standalone pipeline. From an operational point of view, a standalone pipeline could be re-running daily or on schedule.

See also #88

[pombredanne]

@keshav-space ping

[keshav-space]

Implemented in aboutcode-org/scancode.io#788 and aboutcode-org/scancode.io#836