Fix #3940: unload plugin modules on updates

[JohnXLivingston]

Description

Properly unload modules loaded by plugins when updating them.

Related issues

This is a fix for #3940

Has this been tested?

• 👍 yes, I added tests to the test suite
• 👍 yes, light tests as follows are enough
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because they aren't needed
• 🙋 no, because I need help

[JohnXLivingston]

I replaced the call to delete require.cache[...] by a call to decache. It is done in registerPlugin when upgrading a plugin.
Maybe we should also do it when unregistring plugins?

[Chocobozzz]

Thanks, I'll merge this PR manually

[JohnXLivingston]

Thanks, I'll merge this PR manually

Can you wait a moment? I'm going to add decache calls in config reloading code (cf comment on the issue)

[Chocobozzz]

I added it in bae9303

Thanks @JohnXLivingston for the PR and the tests!