Renames and moves the cross validation splitter package.
First, the package and classes are renamed from using
"cross validation splitter" to "train test splitter".
Cross validation as a term is overloaded and encompasses
more concepts than what we are trying to do here.
Second, the package used to be under `process` but it does
not make sense to be there, it can be a top level package
under `dataframe`.
Backport of #59529
ee4610c0ca