Refactor and clarify name relaxation policy changes.
A previous PR introduced "relaxed naming", but mixed up two very safe
true relaxations of constraints with one potentially breaking change,
delaying our ability to land these changes while fixing user code.
We separate the staging flag 'flax_relaxed_naming' into an additional
'flax_preserve_adopted_names' flag to control the potentially breaking
change that preserves module names on adoption.
We enable by default the safe name conflict policy relaxations in this
change.
Both changes are expected to be flipped on and the flags removed in the near
future, but this refactor should make landing everything simpler.
