CLN: Refactor Index._validate_names()

[spacesphere]

Currently Index._validate_names() does not only validation, but some other unrelated to validation stuff:

pandas/pandas/core/indexes/base.py

Lines 803 to 820 in 055bfa6

	def _validate_names(self, name=None, names=None, deep=False):
	"""
	Handles the quirks of having a singular 'name' parameter for general
	Index and plural 'names' parameter for MultiIndex.
	"""
	from copy import deepcopy
	if names is not None and name is not None:
	raise TypeError("Can only provide one of `names` and `name`")
	elif names is None and name is None:
	return deepcopy(self.names) if deep else self.names
	elif names is not None:
	if not is_list_like(names):
	raise TypeError("Must pass list-like as `names`.")
	return names
	else:
	if not is_list_like(name):
	return [name]
	return name

Moreover, it's closely tied in with copying logic without any reason, so that _validate_names() cannot be used in other cases.
I think it would be reasonable to separate these responsibilities.

[gfyoung]

@PoppyBagel : Thanks for reporting this! Perhaps the name is a little misleading, as you're indeed correct that we don't just validate. Maybe a name like "_parse_names" might be more appropriate. Feel free to propose another name in a PR if you like.

[jbrockmendel]

Probably becomes simpler once #44916 is enforced.