Plugins: Access to current sources before `InjectSourceEarly`

I've run into an issue while creating a plugin.

My plugin needs to inject sources, however, it has to dynamically figure out what is missing from the current sources. This is an issue since the `InjectSourceEarly` method does not provide access to either the config or the current sources.

One way would have been to read the config at an earlier point, but the `EarlySourceInjector` plugins are the first to run.

### Suggestion

Have a `ConfigCompleter` or `ConfigExtender` hook for plugins to be able to read the config **before** any generation starts. The `MutateConfig` hook runs too late. Or maybe split `ConfigMutator` into `EarlyConfigMutator` and `LateConfigMutator`

OR

Pass the existing sources to the `EarlySourceInjector` and `LateSourceInjector` plugins. Plugin that needs to change what is injected based on what is already present would appreciate this.