Fix #3555: Let plugin name be correctly accessed.

[vsajip]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

Get the plugin name from the correct source.

[Kwpolska]

Is this change backwards-compatible? Are any plugins (in core or in plugins.getnikola.com) broken by this change?

[vsajip]

I ran the tests locally and they gave the same results with and without this change (some xfails) but I didn't specifically check with all plugins in plugins.getnikola.com. I came across this because without this change the existing code doesn't actually pick up the name from where it should, and so IMO behaves contrary to expectations.