codeintel: add dependency repositories referenced in LSIF uploads

Created by: Strum355

Previously, Maven dependencies had to be manually configured in the external services settings (either via UI or via the JSON config file). This PR adds a (mostly) generic process through which external packages/dependencies that are referenced in an LSIF upload can be added as repositories in Sourcegraph via their relevant external service.

For example: When an LSIF file is uploaded that contains a package reference with the maven/com.sourcegraph/library moniker name, the moniker scheme, name and version are stored in a separate table. Any 'JVM Dependencies' external services are marked to sync, upon which repo-updater JVM Dependencies repo sources can query the table for all package references with the semanticdb scheme, query maven artifact hosts for the package, and create a repo in the repo table for them.

Caveat: while in the same flow as auto indexing dependencies, in its current stage, new to-be-added repositories wont have an autoindex job, as the repo must first exist for a job to be created for it.