Remove unintentional GeoDjango re-exports

[intgr]

Follow-up from discussion on #1299.

Stub files with from ... import SomeSymbol as SomeSymbol are explicit re-exports (https://typing.readthedocs.io/en/latest/source/stubs.html#imports)

Django does employ intentional re-exports to make imports shorter and more convenient (usually in __init__.py files). But many of these re-exports were under longer module paths. And these unintentional re-exports are likely to be less stable over Django releases. E.g.

# Instead of:
from django.contrib.gis.geos.geometry import hex_regex
from django.contrib.gis.sitemaps.kml import Sitemap
# Use:
from django.contrib.gis.geometry import hex_regex
from django.contrib.sitemaps import Sitemap

Now mypy will warn users when using these re-exports.

[intgr]

@ciscorn would be great if you could take a look at this PR.

[ciscorn]

@intgr Looks good to me. We shouldn't rely on these re-exports.

[christianbundy]

this is great, thank you!