ModuleHelper variables management (#2162)

* added metadata for variables in module helper

* adjustments

* added separate support for tracking changes

* rewrote the diff code

* added integration test for module_helper

* using ansible.module_utils.common.dict_transformations.dict_merge

* improved dependency management

* restore ModuleHelper to base classes of CmdStateModuleHelper

* added assertions to ensure the failing module name appears in the error messages

* added test code for state-based modules

* fixed test name

* renamed class to VarMeta

* small fixes

* fixes from the PR

* fixed VarDict.__set_attr__

* added VarDict.__getitem__()

* added changelog fragment

* adjustments per PR

* ModuleHelper.output is now aware of conflicting variable names

* Update plugins/module_utils/module_helper.py

Co-authored-by: Felix Fontein <felix@fontein.de>

Co-authored-by: Felix Fontein <felix@fontein.de>

tests/integration/targets/module_helper/tasks/msimple.yml

@@ -0,0 +1,54 @@
+- name: test msimple 1
+  msimple:
+    a: 80
+  register: simple1
+
+- name: assert simple1
+  assert:
+    that:
+      - simple1.a == 80
+      - simple1.abc == "abc"
+      - simple1.changed is false
+      - simple1.value is none
+
+- name: test msimple 2
+  msimple:
+    a: 101
+  ignore_errors: yes
+  register: simple2
+
+- name: assert simple2
+  assert:
+    that:
+      - simple2.a == 101
+      - 'simple2.msg == "Module failed with exception: a >= 100"'
+      - simple2.abc == "abc"
+      - simple2.failed is true
+      - simple2.changed is false
+      - simple2.value is none
+
+- name: test msimple 3
+  msimple:
+    a: 2
+    b: potatoes
+  register: simple3
+
+- name: assert simple3
+  assert:
+    that:
+      - simple3.a == 2
+      - simple3.b == "potatoespotatoes"
+      - simple3.c == "NoneNone"
+      - simple3.changed is false
+
+- name: test msimple 4
+  msimple:
+    c: abc change
+  register: simple4
+
+- name: assert simple4
+  assert:
+    that:
+      - simple4.c == "abc change"
+      - simple4.abc == "changed abc"
+      - simple4.changed is true