Check typing for PluginMount.plugins in plugin.py

[dschristianson]

My IDE is complaining that the plugin type is a str instead of DataSourcePluginPoint. Therefore in the lines below that call the method plugin.get_meta() is not valid because it thinks plugin is a str.

In the plugin.py module, the PluginMount.plugins element (line 224) is defined as a Dict[DataSourcePluginPoint, str]. So it seems like potentially, the for loop here should be for plugin, name in PluginMount.plugins.items().

BUT none of our tests are failing which means the catalog is being initialized correctly. Not sure what is going on.

Originally posted by @dschristianson in #201 (comment)