Package import should merge same package occurrence

[sgalles]

How to reproduce :
In the same package, create 3 classes in 3 files :

shared class Foo(Bar b, Baz c) {} // file 1
shared class Bar() {} // file 2
shared class Baz() {} // file 3

Create an other package, and simultaneously drag and drop files of Bar and Baz into an other package
you get :

import mymod.sub {
    Bar
}
import mymod.sub { // ERROR : duplicate import: 'mymod.sub'
    Baz
}
shared class Foo(Bar b, Baz c) {}