Fix legacy plugin loading for packages

[avaris]

Resolves: #2805

• Resolves packages for legacy plugins (i.e. "foo.bar") and adds tests for it
• Restores pelican.signals but it will raise an ImportError. So, plugins using import pelican.signals directly will see this:

  Importing from pelican.signals is deprecated. Use from pelican import signals or import pelican.plugins.signals instead.

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• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
  Updated documentation for changed code

[justinmayer]

Really appreciate you taking the time to work on solutions for these issues. You rock! 🥇