Expand Apex Completion for Default Org

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Describe the solution you'd like
Currently Apex completion only works for global (SF) classes and those stored locally in source. A lot of the time you need to reference other classes, however, and it would be nice to have autocompletion on these if a default org is configured.

Ideally, we would be able to explicitly declare the namespaces to retrieve for autocompletion - via the json config file or other means - and have only those namespaces pulled in.