REF/DEPR: DatetimeIndex constructor
This is preliminary to implementing DatetimeArray._from_sequence.  The attempt to implement that directly (i.e. without existing circularity) failed, so trying it as a 2-parter.

Similar to #23539, this deprecates the option of passing timedelta64 data to DatetimeIndex.

There are a few places where I've made TODO notes of the "is this really intentional?" form; I'll point this out in inline comments.

@mroeschke I edited a couple of docstrings for the recently-implemented "nonexistent" kwarg, can you confirm that I didn't mess them up?