Plugins must define app_name in api/urls.py to be included in API root view

[jeremystretch]

Environment

• Python version: 3.6.9
• NetBox version: 2.9.7

Steps to Reproduce

I'm actually not certain whether this is better classified as a bug or a lapse in the documentation, but I'm opting to propose it as a bug because I think it can be addressed by a code change.

1. Create/install a plugin which provides at least one REST API endpoint. (Assume the plugin's name is myplugin.)
2. Within api/urls.py, set app_name = myplugin-api
3. Visit /api/plugins/ in the browser. The plugin should be included.
4. Remove the app_name definition from api/urls.py.
5. Visit /api/plugins/ in the browser again. The plugin no longer appears.

Expected Behavior

Plugin API endpoints should be detected automatically. AFAICT there's no reason a plugin should need to declare app_name. (The core apps do this, but only because they're being included dynamically.)

Observed Behavior

Plugin API endpoints disappear from the list when app_name is not declared.

It seems like the need for the app_name variable could be obviated within PluginsAPIRootView._get_plugin_entry() by changing

api_app_name = import_object(f"{plugin}.api.urls.app_name")

to

api_app_name = f'{app_config.name}-api'

This actually raises another point: the full URL name for a plugin API endpoint is currently in the format plugins-api:myplugin-api:url-name, which seems a bit unwieldy: plugins-api:myplugin:url-name should suffice. However, this would be a breaking change for any plugins which rely on reverse URL resolution to their REST API endpoints.