Simplify custom component loading

[balloob]

Breaking change

Proposed change

When loading custom components, we would jump twice into the executor right after one another. This rewrites the method to be fully run in the executor.

I want this reviewed by @bdraco as the import code has special conditions.

It also moves one more I/O (import custom_components) to the executor.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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Checklist

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[bdraco]

python/cpython#83065 shouldn't be an issue here since we import this in bootstrap