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/runc/tasks/main.yml

@@ -30,11 +30,11 @@
 - name: Runc | Download runc binary
   include_tasks: "../../../download/tasks/download_file.yml"
   vars:
-    download: "{{ download_defaults | combine(downloads.runc) }}"
+    download: "{{ download_defaults | combine(downloads_file.runc) }}"
 
 - name: Copy runc binary from download dir
   copy:
-    src: "{{ downloads.runc.dest }}"
+    src: "{{ downloads_file.runc.dest }}"
     dest: "{{ runc_bin_dir }}/runc"
     mode: "0755"
     remote_src: true