Don't run analyses if plugin is not in the `plugins` section

[alilleybrinker]

Currently, if a user specifies a top-level plugin in their analyze block that isn't listed in the plugins block, but the plugin itself is found in the local plugin cache (for example if it was used previously), Hipcheck will still try to run it.

We shouldn't do this, as it can lead to surprising cases where a user believes they've removed a plugin but actually haven't.

Instead, we should produce an error message saying to either add the plugin to the plugins section (if they intend to use it) or to remove it from the analyze section if they intended to remove it.