Generate requirements_* from manifests (#22718)

## Description:
Generate requirements_* from manifests (if present). If not, fallback to the current approach of reading `REQUIREMENTS` from the module attribute. I disabled exploring the children of the `homeassistant.components.*` packages since that will just add a dependency (from the manifest) due to each of the python files in the package. Just having one for the top level package should be sufficient.

**Related issue (if applicable):** relates to #22700 

## Checklist:
  - [x] The code change is tested and works locally.
  - [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
  - [x] There is no commented out code in this PR.

[ex-requir]: https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/components/keyboard/__init__.py#L14
[ex-import]: https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/components/keyboard/__init__.py#L23


Co-authored-by: Jason Hu <awaregit@gmail.com>

script/manifest/requirements.py

@@ -0,0 +1,22 @@
+"""Helpers to gather requirements from manifests."""
+from .manifest_helper import iter_manifests
+
+
+def gather_requirements_from_manifests(process_requirements, errors, reqs):
+    """Gather all of the requirements from manifests."""
+    for manifest in iter_manifests():
+        assert manifest['domain']
+
+        if manifest.get('requirements') is None:
+            errors.append(
+                'The manifest for component {} is invalid. Please run'
+                'script/manifest/validate.py'.format(manifest['domain'])
+            )
+            continue
+
+        process_requirements(
+            errors,
+            manifest['requirements'],
+            'homeassistant.components.{}'.format(manifest['domain']),
+            reqs
+        )