REF: Create _lib/window directory
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

xref https://github.com/pandas-dev/pandas/pull/29428#issuecomment-555882765

Puts `pandas/_libs/window.pyx` and `pandas/_libs/window_indexer.pyx` into their own directory as
`pandas/_libs/window/aggregations.py` and `pandas/_libs/window/indexers.pyx` respectively.