use npy_datetimestruct and NPY_DATETIMEUNIT instead of pandas versions
pandas implements `pandas_datetimestruct` and `PANDAS_DATETIMEUNIT`, AFAICT because at the time they were implemented, pandas was supporting versions of numpy that did not yet have `npy_datetimestruct` and `NPY_DATETIMEUNIT`.  The implementations are identical.

The upshot of using the numpy versions is dependency simplification.  There are a handful of functions/modules in `tslibs` (also `tslib`, `_libs.period`) that depend on `tslibs.np_datetime` and as a result need to have `tseries_depends` and `np_datetime_sources` in their extension specification.  In many cases, this is including much more than is necessary (and while I haven't timed it, I assume this impacts the build speed).

By using the numpy versions, we can import just datetimestruct and DATETIMEUNIT in the relevant places, avoiding dependencies on src/datetime/, lightening the build, and simplifying setup.py.

Note this PR doesn't actually _do_ any of those simplifications, is just the preliminary search+replace.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
