REF: pandas/core/window.py into multiple files
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Splits `pandas/core/window.py` into more logical division of:

`pandas/core/window/rolling.py`
`pandas/core/window/expanding.py`
`pandas/core/window/ewm.py`
`pandas/core/window/common.py`

Doesn't need to be part of this PR, but could use your input: I've been wondering if it makes sense to put window and resample into groupby/, or otherwise organize in a way to denote that those two modules depend on groupby/ @jbrockmendel definitely. I imagine a lot of the aggregation functions could be collapsed as well if rolling, resample, and groupby lived under the same umbrella. > Doesn't need to be part of this PR, but could use your input: I've been wondering if it makes sense to put window and resample into groupby/, or otherwise organize in a way to denote that those two modules depend on groupby/

this would be too much currently: each of these is already a tremendous amount of tests and code
nesting more doesn’t help Note that #27767 was just merged so need to account for that in rebase yep this needs a rebase, ping on green. Ping all green. thanks @mroeschke 

did we have an issue for this? if so pls close it