Import [SPACE] should auto-pop up list of possible imports

[MikhailArkhipov]

Currently Import [SPACE] does not show list of imports while Control+Space brings it up. Would be good to pop it up automatically

[brettcannon]

Since the stdlib itself has over 100 modules, the initial list will be rather long. And after a single letter the list already pops up and gets short enough to be manageable.