* Disallow importing non-dask estimators from xgboost.dask
This is mostly a style change, but also avoids a user error (that I have
committed on a few occasions). Since `XGBRegressor` and `XGBClassifier`
are imported as parent classes for the `dask` estimators, without
defining an `__all__`, autocomplete (or muscle) memory will produce the
following with little prompting:
```
from xgboost.dask import XGBClassifier
```
There's nothing inherently wrong with that, but given that
`XGBClassifier` is not `dask` enabled, it can lead to confusing behavior
until you figure out you should've typed
```
from xgboost.dask import DaskXGBClassifier
```
Another option is to alias import the existing non-dask estimators.
* Remove base/iter class, add train predict funcs
