feat: use top-level key for repo type instead of type field

Replace the flat `remotes:` map (with `type: "remote"/"virtual"/"local"`) with
separate top-level sections — `remote:`, `virtual:`, `local:` — so the repo
type is declared structurally and the `type:` field is no longer needed.

Config loader normalises the new format to the existing internal representation
(injecting `type` into each remote dict), so all handler code is unchanged.
Adds a TestYamlTypeKeys suite covering all three type keys, mixed files, and
field preservation. Includes README migration guide for splitting a single
remotes file into per-type-and-package conf.d files.

examples/conf.d-method/alpine.yaml

@@ -1,7 +1,6 @@
-remotes:
+remote:
   alpine:
     base_url: "https://dl-cdn.alpinelinux.org"
-    type: "remote"
     package: "alpine"
     description: "Alpine Linux APK package repository"
     immutable_patterns:

examples/conf.d-method/github.yaml

@@ -1,7 +1,6 @@
-remotes:
+remote:
   github:
     base_url: "https://github.com"
-    type: "remote"
     package: "generic"
     description: "GitHub releases and files"
     immutable_patterns:

examples/conf.d-method/pypi.yaml

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