Consider moving package manager api files to internal directory

Summary

Consider moving package management APIs into ee/lib/api/internal

Improvements

It is expected that anything in ee/lib/api would be part of the public v4 API directly consumable by the mechanism described in https://gitlab.com/gitlab-org/gitlab/blob/6214f77456eee3f6620ae2f548ae1bf68a9c4141/doc/api/README.md#basic-usage

As these endpoints aren't intended to be directly consumable (they are meant to be consumed by package manager clients, not directly by users necessarily), they don't fit in with existing API behavior.

Risks

If the internal API is only meant for GitLab services communicating with one another, then this may not be the correct location for these API endpoints since they are called by external services (users machines, CI, etc)

Involved components

ee/lib/api/conan_packages.rb ee/lib/api/maven_packages.rb ee/lib/api/npm_packages.rb

References

This issue was raised in !17797 (comment 241132033)