REF: do all casting _before_ call to DatetimeEngine.get_loc
Fixes: incorrectly letting integers or mismatched types through.

Removes: maybe_datetimelike_to_i8, along with the recently-removed _to_M8 and he hopefully soon-removed pydt_to_i8 (xred #30854) i think we'll be rid of our kludgy datetime casting functions.

Performance neutral:

```
       before           after         ratio
     [28e909c6]       [890204ca]
     <master>         <cln-maybe_datetimelike_to_i8>
+         459±3μs        694±300μs     1.51  groupby.GroupByMethods.time_dtype_as_field('int', 'tail', 'transformation')
+      1.84±0.2μs         2.38±2μs     1.29  index_cached_properties.IndexCache.time_shape('Float64Index')
+     1.09±0.07μs      1.27±0.09μs     1.16  index_cached_properties.IndexCache.time_inferred_type('Float64Index')
+     1.43±0.01μs       1.63±0.2μs     1.14  tslibs.timestamp.TimestampConstruction.time_fromordinal
+         510±7μs         576±60μs     1.13  ctors.SeriesConstructors.time_series_constructor(<function list_of_str at 0x7fe19dbc09d8>, True, 'int')
+     6.75±0.06μs      7.51±0.05μs     1.11  index_object.Indexing.time_get_loc_non_unique_sorted('Int')
+         517±8μs         573±70μs     1.11  ctors.SeriesConstructors.time_series_constructor(<function list_of_str at 0x7fe19dbc09d8>, True, 'float')
+     4.83±0.06ms      5.33±0.07ms     1.10  index_object.SetOperations.time_operation('int', 'symmetric_difference')
+     1.08±0.09μs      1.19±0.07μs     1.10  index_cached_properties.IndexCache.time_is_all_dates('Float64Index')
+     1.74±0.02ms      1.92±0.04ms     1.10  inference.ToNumericDowncast.time_downcast('datetime64', 'float')
-      25.4±0.8μs       23.1±0.2μs     0.91  indexing.NumericSeriesIndexing.time_getitem_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
-     8.31±0.06ms       7.53±0.2ms     0.91  inference.DateInferOps.time_timedelta_plus_datetime
-      16.1±0.7ms       14.6±0.3ms     0.91  timedelta.ToTimedelta.time_convert_string_days
-      14.0±0.8μs      12.7±0.08μs     0.90  index_object.Indexing.time_get_loc('Float')
-     1.94±0.01ms      1.75±0.02ms     0.90  index_object.Indexing.time_get_loc_non_unique('Int')
-     12.8±0.04ms       11.5±0.1ms     0.90  multiindex_object.GetLoc.time_med_get_loc_warm
-      14.1±0.7μs       12.6±0.1μs     0.89  index_object.Indexing.time_get_loc_sorted('Float')
-     12.5±0.04μs       11.1±0.1μs     0.89  multiindex_object.GetLoc.time_string_get_loc
-      10.2±0.6μs      9.05±0.05μs     0.89  index_object.Float64IndexMethod.time_get_loc
-      12.3±0.1ms       10.8±0.1ms     0.88  multiindex_object.GetLoc.time_small_get_loc_warm
-      34.4±0.6μs       30.2±0.6μs     0.88  index_object.Indexing.time_get_loc_non_unique_sorted('Float')
-         104±3μs         91.0±2μs     0.88  multiindex_object.GetLoc.time_large_get_loc
-      4.90±0.1ms      4.21±0.02ms     0.86  timeseries.ResampleSeries.time_resample('period', '1D', 'ohlc')
-      3.78±0.2ms      3.23±0.01ms     0.85  timeseries.ResampleSeries.time_resample('datetime', '5min', 'mean')
-      10.4±0.6μs      8.74±0.03μs     0.84  tslibs.timedelta.TimedeltaConstructor.time_from_iso_format
-      4.38±0.1ms      3.69±0.02ms     0.84  timeseries.ResampleSeries.time_resample('period', '5min', 'mean')
-      3.23±0.5μs       2.24±0.7μs     0.69  index_cached_properties.IndexCache.time_values('TimedeltaIndex')
-       641±200μs          443±1μs     0.69  groupby.GroupByMethods.time_dtype_as_field('int', 'head', 'direct')
-        2.73±1μs       1.45±0.2μs     0.53  index_cached_properties.IndexCache.time_values('UInt64Index')

```