Add option to choose app when running management command

[Yuri Escalianti]

In Django 3.1.2, if two apps have a management command with the same name, Django runs the one which belongs to the app that appears first in INSTALLED_APPS.
This is not an issue, unless the developer does not want to change the order of INSTALLED_APPS. In this case the developer has to workaround the conflict by:

1. Overriding the management command and placing it inside another app with higher precedence in INSTALLED_APPS

or

2. Changing other parts of the code that would be affected by changing the order of INSTALLED_APPS

To remove the need for those workarounds, I suggest adding an option to choose which app to use (therefore bypassing the INSTALLED_APPS order) when running the management command:

python manage.py --app myapp my_management_command

This would make Django run my_management_command from the app myapp.

[Mariusz Felisiak]

Duplicate of #27189.