Remove redundant vars FA on PlaybookInclude (#85395)

* The redundant FA declaration was not static, which broke a number of automatic validation behaviors.
* Added tests to assert deferred validation and lack of templating on `import_playbook.vars`.

Co-authored-by: Matt Clay <matt@mystile.com>

test/integration/targets/include_import/playbook/playbook_using_a_var.yml

@@ -0,0 +1,6 @@
+- hosts: localhost
+  gather_facts: no
+  tasks:
+    - name: Verify an imported playbook can see a var it was given
+      assert:
+        that: pb_var == 'hello'

test/integration/targets/include_import/playbook/test_import_playbook.yml

@@ -20,3 +20,15 @@
 
 # https://github.com/ansible/ansible/issues/59548
 - import_playbook: sub_playbook/sub_playbook.yml
+
+- name: Use set_fact to declare a variable
+  hosts: localhost
+  gather_facts: no
+  tasks:
+    - set_fact:
+        a_var_from_set_fact: hello
+
+- name: Verify vars for import_playbook are not templated too early
+  import_playbook: playbook_using_a_var.yml
+  vars:
+    pb_var: "{{ a_var_from_set_fact }}"