PR: Find internal plugins in plugin module for Spyder installs

[goanpeca]

Description of Changes

Issue(s) Resolved

• This PR correctly finds internal plugins by checking in the spyder.plugins folder for installs running outside the normal spyder tests environment. This is needed to run Spyder with other plugins like the ATE one
• PLUGIN_CLASS is replaced with PLUGIN_CLASSES as one plugin folder could potentially expose more than 1 plugin. Now that entry points are the way to expose plugins, this PLUGIN_CLASS is no longer needed for external plugins.

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 2020-07-27 20:02:44 UTC

[goanpeca]

@nerohmot, once this is merged, the tests against the Spyder application in Semi-ATE/Semi-ATE#10 can pass.

[nerohmot]

👍 @ccordoba12 When this PR is in, we can test Semi-ATE against spyder! 🎉

[ccordoba12]

Looks good to me, thanks @goanpeca!

[ccordoba12]

When this PR is in, we can test Semi-ATE against spyder!

Great news @nerohmot!