Inline shared methods from ::API::Helpers::Packages::Npm to the endpoints

This is a follow-up to Refactor NPM metadata endpoint

In that issue we:

• inlined the metadata endpoint from the shared concern into the project metadata endpoint
• moved the metadata endpoint from the shared concern into a new module, shared by both the group and instance endpoints

As a next step, we'll inline the shared methods in ::API::Helpers::Packages::Npm into the individual endpoint files, lib/api/npm_group_packages.rb, lib/api/npm_instance_packages.rb and lib/api/npm_project_packages.rb.