move all uses of metadata to the corresponding output unit

[sigmundch]

Historically all metadata emitted for mirrors and Function.apply has been emitted in the main output unit.

To fix #30002, we are moving metadata of function parameters to the same output unit where the function is emitted.

We should consider doing the same for all other metadata. However, a lot of the additional metadata requires complex changes and, if it is only used for dart:mirrors, we might be able to do have a simpler fix after #30538 is addressed.

[sigmundch]

Closing this - I believe I had misunderstood where the changes needed to happen, and it's possible that this is now handled by the changes in https://dart-review.googlesource.com/9140. We can reopen this bug if we discover any issues regarding this.