[bug] Django and Dynaconf: Can't merge INSTALLED_APPS from external settings_file into settings.py

[Hammit]

Describe the bug
When I have a Django project with Dynaconf integration, I get startup errors.
I have my settings in settings.py and am using Dynaconf in Explicit Mode to merge INSTALLED_APPS from an external json file with the Django defaults in settings.py

Minimal git repo reproducing the bug

To Reproduce
Clone the repo
cd into the project dir
Create the venv (python3 -m venv venv)
Activate the venv (source venv/bin/activate)
Install dependencies (pip install -r requirements.txt)
Run the server (./manage.py runserver)

Expected behavior
To run the server without errors. The result should be that the INSTALLED_APPS from the /django_dynaconf/configs/external_settings.json should be merged with the INSTALLED_APPS setting in django_dynaconf/settings.py

Environment (please complete the following information):

• OS: WSL2 Ubuntu 22.04.3
• Dynaconf Version 3.2.4
• Frameworks in use (Django 5.0.1 and 4.2)

Additional context
I believe I'm forced to use Explicit Mode due to the complexities of the larger project that this minimal reproduction doesn't highlight

Stack Trace

$ ./manage.py runserver
INSTALLED_APPS=['django.contrib.admin', 'django.contrib.auth', 'django.contrib.contenttypes', 'django.contrib.sessions', 'django.contrib.messages', 'django.contrib.staticfiles']
INSTALLED_APPS=['django.contrib.admin', 'django.contrib.auth', 'django.contrib.contenttypes', 'django.contrib.sessions', 'django.contrib.messages', 'django.contrib.staticfiles']
Watching for file changes with StatReloader
Performing system checks...

Exception in thread django-main-thread:
Traceback (most recent call last):
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/apps/registry.py", line 158, in get_app_config
    return self.app_configs[app_label]
KeyError: 'admin'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.10/threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/utils/autoreload.py", line 64, in wrapper
    fn(*args, **kwargs)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/core/management/commands/runserver.py", line 133, in inner_run
    self.check(display_num_errors=True)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/core/management/base.py", line 485, in check
    all_issues = checks.run_checks(
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/core/checks/registry.py", line 88, in run_checks
    new_errors = check(app_configs=app_configs, databases=databases)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/core/checks/urls.py", line 42, in check_url_namespaces_unique
    all_namespaces = _load_all_namespaces(resolver)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/core/checks/urls.py", line 61, in _load_all_namespaces
    url_patterns = getattr(resolver, "url_patterns", [])
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/utils/functional.py", line 47, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/urls/resolvers.py", line 735, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/utils/functional.py", line 47, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/urls/resolvers.py", line 728, in urlconf_module
    return import_module(self.urlconf_name)
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/django_dynaconf/urls.py", line 21, in <module>
    path('admin/', admin.site.urls),
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/utils/functional.py", line 251, in inner
    self._setup()
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/contrib/admin/sites.py", line 601, in _setup
    AdminSiteClass = import_string(apps.get_app_config("admin").default_site)
  File "/home/user/src/django/django_dynaconf_INSTALLED_APPS/venv/lib/python3.10/site-packages/django/apps/registry.py", line 165, in get_app_config
    raise LookupError(message)
LookupError: No installed app with label 'admin'.

[rochacbruno]

I will take a look this week

[Hammit]

A workaround I have is to not merge INSTALLED_APPS from external_settings.json but instead rename INSTALLED_APPS in external_settings.json to INSTALLED_APPS_TO_ADD and do this...

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
] + settings.INSTALLED_APPS_TO_ADD