[Downloader] [p]cog uninstall doesn't remove cog from bot._config.packages

[jack1142]

Command bugs

Command name

[p]cog uninstall

What cog is this command from?

Downloader

What were you expecting to happen?

The bot to unload the cog AND remove it from bot._config.packages

What actually happened?

Bot unloaded the cog, but it didn't remove it from bot._config.packages therefore causing error on next bot start:

Failed to load package cogboard
Traceback (most recent call last):
  File "venv\lib\site-packages\redbot\core\bot.py", line 409, in pre_flight
    await asyncio.wait_for(self.load_extension(spec), 30)
  File "C:\Python37\lib\asyncio\tasks.py", line 442, in wait_for
    return fut.result()
  File "lib\site-packages\redbot\core\bot.py", line 648, in load_extension
    name = spec.name.split(".")[-1]
AttributeError: 'NoneType' object has no attribute 'name'

How can we reproduce this issue?

1. Uninstall loaded cog that was installed through Downloader.
2. Restart bot.