Unify native and non-native valid values for ``Enum``

Unify native and non-native valid values for ``Enum``

Unify behaviour :class:`_schema.Enum` in native and non-native
implementations regarding the accepted values for an enum with
aliased elements.
When :paramref:`_schema.Enum.omit_aliases` is ``False`` all values,
alias included, are accepted as valid values.
When :paramref:`_schema.Enum.omit_aliases` is ``True`` only non aliased values
are accepted as valid values.

Fixes: #6146
Change-Id: I6f40789c1ca56e533990882deadcc6a377d4fc40