Split up yaml loaders into multiple files (#23774)

* Start moving parts of yaml utils to own module

Move parts of yaml loader out of the single large file and start
to create the structure of the yaml loaders in Ansible [0].

[0]: https://github.com/ansible/ansible/tree/devel/lib/ansible/parsing/yaml

* Finish yaml migration, update tests and mocks

  * Move code around to finish the migration
  * Update the mocks so that `open` is patched in
    `homeassistant.util.yaml.loader` instead of
    `homeassistant.util.yaml`.
  * Updated mypy ignores
  * Updated external API of `homeasistant.util.yaml`, see below:

Checked what part of the api of `homeassistant.util.yaml` was actually
called from outside the tests and added an `__ALL__` that contains only
these elements.

Updated the tests so that references to internal parts of the API (e.g.
the yaml module imported into `homeassistant.util.yaml.loader`) are
referenced directly from `homeassistant.util.yaml.loader`.

In `tests/test_yaml.py` the import `yaml` refers to
`homeassistant.util.yaml` and `yaml_loader` refers to `~.loader`.

Future work that remains for the next iteration is to create a custom
SafeConstructor and refers to that instead of monkey patching `yaml` with
custom loaders.

* Update mocks in yaml dumper, check_config

homeassistant/util/yaml/objects.py

@@ -0,0 +1,13 @@
+"""Custom yaml object types."""
+
+
+class NodeListClass(list):
+    """Wrapper class to be able to add attributes on a list."""
+
+    pass
+
+
+class NodeStrClass(str):
+    """Wrapper class to be able to add attributes on a string."""
+
+    pass