[dask] Disallow importing non-dask estimators from xgboost.dask (#7133)
[dask] Disallow importing non-dask estimators from xgboost.dask (#7133)

* 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
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost