Add 2.0-2.3 facts api compat (ansible_facts(), get_all_facts()) (#27294)

* Add 2.0-2.3 facts api compat (ansible_facts(), get_all_facts())

These are intended to provide compatibilty for modules that
use 'ansible.module_utils.facts.ansible_facts' and
'ansible.module_utils.facts.get_all_facts' from 2.0-2.3 facts
API.

Fixes #25686

Some related changes/fixes needed to provide the compat api:

* rm ansible.constants import from module_utils.facts.compat

Just use a hard coded default for gather_subset/gather_timeout
instead of trying to load it from non existent config if the
module params dont include it.

* include 'external' collectors in compat ansible_facts()

* Add facter/ohai back to the valid collector classes

facter/ohai had  gotten removed from the default_collectors
class used as the default list for all_collector_classes by
setup.py and compat.py

That made gather_subset['facter'] fail.

test/integration/targets/gathering_facts/test_gathering_facts.yml

@@ -32,6 +32,7 @@
           - 'ansible_mounts|default("UNDEF_NET") != "UNDEF_HW"'
           - 'ansible_virtualization_role|default("UNDEF_VIRT") != "UNDEF_VIRT"'
 
+
 - hosts: facthost19
   tags: [ 'fact_min' ]
   connection: local
@@ -260,3 +261,18 @@
       assert:
         that:
           - '"{{ ansible_local.testfact.fact_dir }}" == "custom"'
+
+- hosts: facthost20
+  tags: [ 'fact_facter_ohai' ]
+  connection: local
+  gather_subset:
+    - facter
+    - ohai
+  gather_facts: yes
+  tasks:
+    - name: Test that retrieving facter and ohai doesnt fail
+      assert:
+        # not much to assert here, aside from not crashing, since test images dont have
+        # facter/ohai
+        that:
+          - 'ansible_user_id|default("UNDEF_MIN") != "UNDEF_MIN"'