API/CLN: Have toplevel pd.pivot mirror pivot instead of pivot_simple
- [x] closes #22116
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


- Moved `pivot` from `pandas/core/reshape/reshape.py` to `pandas/core/reshape/pivot.py`
- Moved `pivot_simple` and `_slow_pivot` from `pandas/core/reshape/reshape.py` to `pandas/core/reshape/pivot.py` (only used by one panel test)
- Remapped top level `pd.pivot` to use `pivot` instead of `pivot_simple` 
- Added some additional tests for `pd.pivot`
