downloads: split file and containers (#13581)

Splits the definitions into two dicts, while joining then for now to
preserve the downloads dict.

Update all consumers of the variables, which are roughly:
- manual includes of the download role (which will be removed in the
  short future)
- install tasks using the resolved 'dest' to find the artefacts

This will make further division of the downloads role easier.

roles/container-engine/kata-containers/tasks/main.yml

@@ -2,11 +2,11 @@
 - name: Kata-containers | Download kata binary
   include_tasks: "../../../download/tasks/download_file.yml"
   vars:
-    download: "{{ download_defaults | combine(downloads.kata_containers) }}"
+    download: "{{ download_defaults | combine(downloads_file.kata_containers) }}"
 
 - name: Kata-containers | Copy kata-containers binary
   unarchive:
-    src: "{{ downloads.kata_containers.dest }}"
+    src: "{{ downloads_file.kata_containers.dest }}"
     dest: "/"
     mode: "0755"
     owner: root