Supporting nested library imports via `-iv`

[rasbt]

Currently, nested imports do not seem to work:

1)

In [1]: from sklearn.model_selection import train_test_split

In [2]: %load_ext watermark

In [3]: %watermark -iv

2)

and only the current scenario is supported (among other variants):

In [4]: import sklearn.model_selection

In [5]: %watermark -iv
sklearn   0.20.2

It would be nice if we could also get 1) to work.