MAINT: Use __module__ in _DeprecatedModule.
State: closed

Follow-up to #14105. Uses the `__module__` method to correctly determine the location of the alternative
module to use.


Labels:
name: Clean

Comments:
## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14181?src=pr) is 85.25% (diff: 86.66%)

> Merging [#14181](https://codecov.io/gh/pydata/pandas/pull/14181?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.01%**

``` diff
@@             master     #14181   diff @@
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  Files           140        140          
  Lines         50555      50544    -11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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- Hits          43095      43091     -4   
+ Misses         7460       7453     -7   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8af6264...2e581ab](https://codecov.io/gh/pydata/pandas/compare/8af626474f6f314527a9ad3f15403aa2dd8c402d...2e581ab768b62788edc178230915bdc168afe8a8?src=pr)

somethings wrong. so now `dir(pandas.datetools)` works, but

`from pandas.core.datetools import yearEnd` does not show a warning

nor does tab completion on the module

```
In [1]: from pandas.core.datetools import /Users/jreback/miniconda/lib/python2.7/site-packages/IPython/core/completerlib.py:158: FutureWarning: The pandas.core.datetools module is deprecated and will be removed in a future ver.
  m = __import__(mod)               

                         ABCDataFrame             BDay                     BMonthEnd                businessDay              cache_readonly           CDay                     CustomBusinessHour       DateParseError            
                         ABCIndexClass            bday                     BQuarterBegin            BusinessHour             CBMonthBegin             cday                     customBusinessMonthBegin datetime                  
                         ABCSeries                bmonthBegin              BQuarterEnd              BYearBegin               cbmonthBegin             compat                   customBusinessMonthEnd   Day                      >
                         algos                    BMonthBegin              bquarterEnd              BYearEnd                 CBMonthEnd               customBusinessDay        D_RESO                   day                       
```

can you add some tests for this

@jreback : Did tab completion work before?

yes

@jreback : I couldn't get tab completion to issue a warning like that on `master`.  Still not sure why the `pandas.core.datetools` import stopped issuing warnings.  I think it is because I imported the module once during `__init__` with a filter.

wel it needs to be the same as master and have the warning and have the same behavior as 0.18.1 for function

@jreback : Fair enough.  I made some modifications.  Can you take a look?

tab complete / warnings loooking good

```
In [1]: import pandas

In [2]: from pandas.core.datetools import C:\Anaconda\envs\pandas3.5\lib\site-packages\IPython\core\completerlib.py:158: FutureWarning: The pandas.core.datetools module is deprecated and will be removed in a future version. Please use the pandas.tseries module instead.
  m = __import__(mod)

In [3]: from pandas.core.datetools import yearEnd
```

```
In [1]: pd.datetools.yearEnd
C:\Anaconda\envs\pandas3.5\Scripts\ipython-script.py:1: FutureWarning: pandas.core.datetools.yearEnd is deprecated and will be removed in a future version.
  if __name__ == '__main__':
Out[1]: <YearEnd: month=12>
```

lgtm. @jorisvandenbossche 

@gfyoung Thanks a lot!