Add support for subpackages (like pulp_file)

[pedro-psb]

This adds support for using subpackages (nested packages under a main repository, like pulp_file) as regular repositories

About subpackages content in the repo:

• Content should live in main-repo/subpackage, e.g. pulpcore/pulp_file/staging_docs/...

About subpackages declaration in the repolist.yml file:

• They contain a special attribute subpackage_of: <reponame>
• <reponame> should be declared as a regular repository
• Should use only the set of attributes {name, title, subpackage_of}.
• The order doesn't matter

An example:

repos:
  content: 
    - name: pulp_certguard # subpackage entry
      title: Certguard
      subpackage_of: pulpcore
  core: 
    - name: pulpcore # regular repository entry
      owner: pulp
      title: Pulp Core
      branch: main