Prevent partial custom component overlays

[balloob]

Description:

This will force platforms to be resolved based off the component path, if it exists.

Today, if you want, you can override the Hue light platform, but not the other parts of the Hue integration. If a future update evolves the Hue component, removing or changing internal methods that the custom platform relied upon, the custom platform will start failing (like this report).

To avoid this, we're going to no longer allow custom components to be partial overlays (just a platform). Instead, if you want to override a built-in platform, you will need to override the whole component.

This is enforced by first resolving the platform as a component, and if it exists, limiting the lookup path to the component path.

Example: if I look up the hue component, and it is provided by a custom component, then all platform lookups will also be looked up in the custom component dir. The same works the other way around, if a user would only try to override hue/light.py but not hue/__init__.py, the custom platform will be ignored.

Breaking change: Custom platforms that override a built-in platform that has a component, should now also include the component in the custom_components/ folder. So if you want to override hue/light.py with a custom version, you will also need to provide/copy over hue/__init__.py.

Related issue (if applicable): fixes home-assistant/architecture#141

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.