Improve plugin testing (#3517)

* refactor entrypoint into helpers

* Add lookup by metadata
This is geared towards plugins packaged in pkgs that differ in name from their top module

* Make module lookup predictable in changing pkg-envs

* remove no coverage from plugin packages

* ignore coverage for production loadin

* refactor plugin collection - move assigment out

* do not cover fs errors

* test custom dir loading

* test module meta fetcher

* add a bit more safety

* do not cover sanity checkers

* add folder loading test

* ignore again for cleaner diffs for now

* ignore safety catch

* rename test

* Add test for package installs

* fix docstring name

* depreciate test for now

* Fix for out of BASE_DIR paths

* ignore catch

* remove unneeded complexity

* add testing for outside folders

* more docstrings and simpler methods

* make call simpler

InvenTree/plugin/apps.py

@@ -43,7 +43,7 @@ class PluginAppConfig(AppConfig):
                         pass
 
                     # get plugins and init them
-                    registry.collect_plugins()
+                    registry.plugin_modules = registry.collect_plugins()
                     registry.load_plugins()
 
                     # drop out of maintenance