REF: de-duplicate code in libperiod
discussed a few months ago, this gets rid of `date_info_from_days_and_time` and instead re-uses `pandas_datetime_to_datetimestruct`
cc @WillAyd trying to figure out which of the current CI failures are false-positives, is this one from Travis a real problem?

```
pandas/_libs/tslibs/period.c:9227:11: error: ‘__pyx_v_factor’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

   __pyx_r = (__pyx_v_sub * __pyx_v_factor);

           ^

pandas/_libs/tslibs/period.c:8995:26: note: ‘__pyx_v_factor’ was declared here

   __pyx_t_5numpy_int64_t __pyx_v_factor;

                          ^

cc1: all warnings being treated as errors

building 'pandas._libs.tslibs.resolution' extension
``` I'm assuming your latest commit fixes the issue, but yea I think just telling you that `factor` could potentially be uninitialized if none of the conditions branch, so either need to guarantee it branches or initialize to a value