PR: Display external plugins in the dependencies dialog

[goanpeca]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #

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: @goanpeca

[pep8speaks]

Hello @goanpeca! 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 2020-07-14 19:07:24 UTC

[goanpeca]

Test failures are unrelated to the work of the PR.

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/m/mesa/libegl-mesa0_19.2.8-0ubuntu0~18.04.3_amd64.deb  404  Not Found [IP: 52.168.50.79 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
##[error]Process completed with exit code 100.

Merging