Alias should allow symbol imports

[benjaminjkraft]

Currently, we only ever generate imports of modules, even if your old import was importing a symbol from a module and you're using auto-alias. We should also have a flag like FROM that says to use symbol imports whereever possible; then FROM would say use module from-imports whereever possible.

(None of this affects KA much, since we generally disallow symbol imports, but the rest of the world uses them surprisingly much!)