Fix issue #1503 with modules and imports sharing a name

[JsonFreeman]

Code copied from bug #1503 for convenience. This fixes #1503.

module Z.M {
    export function bar() {
        console.log(1);
    }
}
module A.M {
    import M = Z.M;
    export function bar() {
        console.log(2);
    }
    M.bar(); // Calls the wrong function at runtime, works if you change the above import to a var
}

The issue is that we were not generating a unique name _M for the module A.M. We need to do this because of the local import M, which we were ignoring.

[yuit]

Could you add test case for when you don't reference the import.

[yuit]

lgtm 👍 About the test case 3 and case 5, I don't mind the fact that they are different as these are emit output from error-ed file