Follow-up from "Adapt Conan API to consider the package registry access level"

The following discussion from !104589 (merged) should be addressed:

• @10io started a discussion: (+1 comment)

  This service is starting to do way too many things to my tasting:

  • Handle single / multiple packages queries.
  • For multiple packages, handle project scoped or not queries.

  I would suggest spliting this in 2:

  • One logic for getting single packages and one for searching packages. I think I would even suggest using 2 different finders: Conan::PackageFinder and Conan::PackagesFinder
  • For the logic getting multiple packages, accept a project: parameter that is nil by default to clearly show that it is nilable.

  This is outside of the scope of this MR. I'll open a follow up issue.