Rename the module to django_adminplus

[jsocol]

Similar to jsocol/django-ratelimit#214, in the year of the Barbie movie 2023, it would be good behavior with neighbors to rename the module from adminplus to django_adminplus. This won't change the name of the package for the purposes of PyPI, but will change the import path from import adminplus to import django_adminplus. Namespacing the package this way plays more nicely with other packaging ecosystems