cleaned up imports
This is a continuation of #18148 and does the following

-  puts the `reshape` imports from `pandas.core.api` to `pandas.core.reshape.api`
- uses `pandas.core.dtypes.generic.ABCMultiIndex` inplace of `MultiIndex` in `melt.py`
- uses `from pandas import DataFrame` in `melt.py` inside functions
- imports directly from pandas in reshape tests

There is one test failing. Statsmodels imports `get_dummies` from `pandas.core.api` which is no longer possible. It should import directly from pandas.
