RepositoryManager.add_repository does not add plugins #2987

Version and Platform (required):

• Binary Ninja Version: Version 3.0.3267-dev Personal (Build ID ef8dc2f8)
• OS: Ubuntu Linux
• OS Version: 20.04

Bug Description:
I'm trying to test a binary ninja plugin's metadata for the plugin manager to eventually add to the community plugins repo.
However, I cannot setup the plugin to work locally using the pluginmanager module in binary ninja.

Just in order to just test the API, I tried to load the vector35 repository using the pluginmanager, but it does not work. The RepositoryManager.add_repository function returns True but the plugins are not loaded properly.

More details below.

Steps To Reproduce:
I entered the following code into the Binary Ninja python console to try loading the vector35 community repository using the API.

>>> mgr = RepositoryManager()
>>> mgr.add_repository("https://raw.githubusercontent.com/Vector35/community-plugins/master/plugins.json", "test")
True
>>> mgr.repositories
[<community>, <official>, <test>]
>>> mgr.repositories[2].plugins
[]

Notice that mgr.repositories[2].plugins is not populated with the plugins like is expected.

Expected Behavior:
After running add_repository as done in the above code snippet, the manager should be able to load all the relevant plugins, to then be installed/enabled.

Screenshots:
If applicable, please add screenshots here to help explain your problem.

Additional Information:
In case it is relevant, the ~/.binaryninja/repositories/ directory is not updated either after the API call.

(manticore) ctf@ctf:~/.binaryninja/repositories
$ ll
total 16
drwxrwxr-x 4 ctf ctf 4096 Feb 24 14:55 ./
drwxrwxr-x 5 ctf ctf 4096 Feb 23 13:31 ../
drwxrwxr-x 3 ctf ctf 4096 Feb 23 12:37 community/
drwxrwxr-x 3 ctf ctf 4096 Feb 23 12:37 official/