[Observation] Refinements for init, peer macros, and qualified type emissions

[phausler]

Includes fixes for the following:

Ensure type access is qualified to the module name. e.g. when an imported module ALSO defines Observable make sure the macros don't cause ambiguity.

Transition to peer macros instead of arbitrary member. This removes the arbitrary member emission from using @Observable and favors the peer macro instead.

Lift the initializer requirement by utilizing init accessors for fully formed definite initialization. This removes the error case where types need fully defined initialization of properties. This is dependent upon #66283

[phausler]

@swift-ci please test