Simplify v6 distribution material

[wagoodman]

This PR makes adjustments to the v6 schema + supporting code from previous PRs, specifically:

• adjusts DB import to function with both today's and v6 schema automatically
• add application config support to create v6 distribution client and installation curator config objects
• removes the metadata.json file; now there is only the DB file in the distributed tarball
• removes checksum from db.Distribution struct
• makes the latest.json file flat (no nested objects)
• adjusts the vulnerability blob such that "assigner" is plural (matches the type)
• adds CHML severity scheme (will be used by the github provider)

This PR also introduces the ability to import both v6 and v5 schemas at once (since import now functions in both contexts) which requires disabling the static analysis check for cross-import of schemas.