Allow dealing with versionless PURLs

[ctron]

Ingesting OSV documents creates "versionless PURLs" (aka base PURLs). It also adds a entries in the version_range table.

However, I don't see any way to get back the originally ingested OSV document through such PURLs. My expectation would be, that there exists a PURL (versioned or not), which I can use to query the system, receiving the originally ingested OSV documents in return.