Change models to one doc section

In the current master, the models from `astropy.modeling` are each documented in their own section (e.g. `astropy.modeling.polynomial`, `astropy.modeling.powerlaws`, etc.).  This strikes me as less than ideal, because it leads to people doing things like ``from astropy.modeling.polynomial import Chebyshev1D``, while what we really want is for people to do ``from astropy.modeling.models import Chebyshev1D``, so that they don't have to worry about the internal organization of the `modeling` package. This PR corrects that by changing it so that just ``astropy.modeling.models`` is documented, which contains all the same classes, but makes it clearer that it's intended that they use ``astropy.modeling.models``.

There's a bunch of changes in `astropy.sphinx.ext`  because it turns out enabling this required some rather substantial changes to how `automodapi` finds modules.  The details are not terribly important unless you're interested, but even if we end up leaving the `modeling` doc the way it is, the *rest* of this should be included, as it contains some bug fixes.

cc @nden