Packages: hide package host connections behind feature flags

Created by: olafurpg

Previously, any site admin could create a package code host without knowing that this integration is experimental. Now, site admins must enable a feature flag to create JVM/npm/python/rust/go package hosts. This change reduces the risk that a customer accidentally turns on packages causing unexpected issues.

Test plan

Manually ran sg start to validate that the package hosts are no longer visible when creating packages. For some reason, the npm and JVM hosts are still visible and I'm not sure why, I suspect it's because of some local state on my computer 🤔