Complete aliased namespaces

Hi @darwin,

as discussed on Slack, I was testing completions in Dirac and just wanted to open an issue as reminder.
Basically completing something like:

```
(require '[clojure.string :as str])
(str/)
```

Would be super awesome.

Some inspiration can be taken from Cider's completion tooling for Clojurescript [here](https://github.com/clojure-emacs/cljs-tooling). I so like the fact that you don't use nRepl at all for this, and basically you're behaving like a self-hosted repl (lumo, planck...) for completions.

Thanks for Dirac by the way and for all your help! :smile: