pacman: implement change detection for update_cache=true; add cache_updated return value (#4337)

* Implement change detection for update_cache=true. Add cache_updated return value.

* ...

* Make sure pacman --sync --list is called only as often as necessary.

tests/integration/targets/pacman/tasks/main.yml

@@ -10,3 +10,4 @@
     - include: 'basic.yml'
     - include: 'package_urls.yml'
     - include: 'remove_nosave.yml'
+    - include: 'update_cache.yml'

tests/integration/targets/pacman/tasks/update_cache.yml

@@ -0,0 +1,23 @@
+---
+- name: Make sure package cache is updated
+  pacman:
+    update_cache: true
+
+- name: Update package cache again (should not be changed)
+  pacman:
+    update_cache: true
+  register: update_cache_idem
+
+- name: Update package cache again with force=true (should be changed)
+  pacman:
+    update_cache: true
+    force: true
+  register: update_cache_force
+
+- name: Check conditions
+  assert:
+    that:
+      - update_cache_idem is not changed
+      - update_cache_idem.cache_updated == false
+      - update_cache_force is changed
+      - update_cache_force.cache_updated == true