Fix Windows handlers definition (#349)

Dynamically includes the Windows Agent restart handler to make it work as expected.

handlers/main-win.yml

@@ -1,8 +1,11 @@
 ---
-
-- name: restart datadog-agent-win
+# This file doesn't actually contain "handlers" in the Ansible sense: when running
+# our role, Ansible only loads the contents of handlers/main.yml as handlers.
+# However, this is here because this is a "handler-like" task that is dynamically
+# included by a handler task in handlers/main.yml.
+- name: Restart Windows datadogagent service
   win_service:
     name: datadogagent
     state: restarted
     force_dependent_services: true
-  when: datadog_enabled and not ansible_check_mode
+  when: datadog_enabled and not ansible_check_mode and ansible_facts.os_family == "Windows"

handlers/main.yml

@@ -12,5 +12,12 @@
     state: restarted
   when: datadog_enabled and not ansible_check_mode and not ansible_facts.os_family == "Windows"
 
-- include_tasks: handlers/main-win.yml
-  when: ansible_facts.os_family == "Windows"
+# We can't add the Windows Agent service restart handler directly here because that makes the role require
+# the ansible.windows collection on all platforms. We only want it to be needed on Windows.
+# Therefore, what we do is the following: when needed, our Windows tasks call this handler to require a
+# Windows Agent restart (through notify: restart datadog-agent-win).
+# When notified, the below handler is executed at the end of the playbook run.
+# The include_tasks loads the handlers/main-win.yml file, which contains the real service restart task
+# (which depends on ansible.windows), and runs it, triggering the Windows Agent restart.
+- name: restart datadog-agent-win
+  include_tasks: handlers/main-win.yml