codeintel: dont attempt syncing non-package-repo dependencies outside of dotcom

Created by: Strum355

Existing behaviour: originally designed for dotcom, where repos from github n co (aka cloud_default external services) are lazily fetched, we would make repo-updater sync a potentially new repo by performing a RepoLookup for package references that didnt denote a package repo external services (aka for Go package references, as theyre git-hosted).

This is undesirable outside of dotcom (where there are no cloud_default external services), as it results in alerts and logs on repos failing to sync.

New behaviour: we now specifically check if we're in dotcom mode and only lookup repos via repo-updater when 1) we're in dotcom mode and 2) the repo does not already exist in the repo table

Closes #40517 (closed)

Test plan

Created new and updated existing unit tests to check for specific repo-updater function calls