Flatten namespace

I think that in order to use the `imread` function I need to dive within the imread module today

```python
import dask_image.imread

x = dask_image.imread.imread('...')
```

This might be more user friendly if it was just

```python
import dask_image

x = dask_image.imread('...')
```

Separately, it also looks like this is the only function in the `imread` module. I wonder if maybe having a seaprate module here is overkill for now?

Inspect the public dask_image and dask_image.imread entry points to determine how imread is exposed today. Decide whether the flattened import should be supported and whether the separate module remains; done means the chosen API works consistently and its behavior is covered by the project’s existing checks.

Written by the indexing model from the issue text.