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/molecule/prepare.yml

@@ -16,7 +16,7 @@
     - name: Download CNI
       include_tasks: "../../download/tasks/download_file.yml"
       vars:
-        download: "{{ download_defaults | combine(downloads.cni) }}"
+        download: "{{ download_defaults | combine(downloads_file.cni) }}"
 
 - name: Prepare CNI
   hosts: all