PR: Several fixes to load external plugins

[ccordoba12]

Description of Changes

This PR fixes several problems we discovered in 5.0 alpha4 when loading external plugins:

• Add missing entry_points and fix init files for new plugins added in alpha3 and alpha4.
• Fix solving list of required plugins for external plugins.
• Fix adding third-party toolbars to the toolbars menu.
• Make Preferences a required plugin for a couple of plugins that had it as optional.

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @ccordoba12

[pep8speaks]

Hello @ccordoba12! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-02-21 01:11:20 UTC