completion resolves used names

This was making me think I was crazy for a while today:
```jl
julia> module M
           export blah
           blah = 1
       end
Main.M

julia> using .M

julia> bl<tab> = "other"
ERROR: cannot assign variable M.blah from module Main
Stacktrace:
 [1] top-level scope at none:0
```
Tab completing a name provided by `using` should not resolve it.