Part of #137040 and #80374
- Extracted getting plugin implementation candidates and the default implementation to its own method
- Extracted resolving the plugin implementation to its own method
- Simplify candidate selection algorithm
- Support overriding inline dart implementation for an app-facing plugin
- Throw error, if a federated plugin implements an app-facing plugin, but also references a default implementation
- Throw error, if a plugin provides an inline implementation, but also references a default implementation
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