refactor: split config into remotes/virtuals/locals sections

Repository types now live under dedicated top-level keys instead of a
shared remotes: block distinguished by a type field:

  remotes:   caching proxy remotes (no type field needed)
  virtuals:  virtual merged-index repositories
  locals:    local upload repositories

Routes for local repos move from /api/v1/remote/ to /api/v1/local/.
config.py gains get_virtual_config() and get_local_config() lookups.
Root endpoint now reports all three sections. Drop root conf.d/ (was
an exact duplicate of examples/conf.d-method/).

examples/conf.d-method/pypi.yaml

@@ -1,7 +1,6 @@
 remotes:
   pypi:
     base_url: "https://files.pythonhosted.org"
-    type: "remote"
     package: "pypi"
     description: "Python Package Index"
     check_mutable_updates: true