Make auto-completion require/import-aware

[eerohele]

For example, given:

(import '(java.util.concurrent C))

Tutkain probably ought to be smart enough to suggest CountDownLatch.

[eerohele]

Similarly, Tutkain should probably suggest only Clojure namespaces when requiring and only Java classes/interfaces when importing. Ought to consider a general approach to conveying auto-completion context over the backchannel.

[eerohele]

Will be included in 0.20.0.