Fix verification of import reassignments

* When verifying reassignments are not imports, make sure the identifier matches an import.
* When verifying re-declarations of imports, make sure the scope is global
does this covers this case?

```
import { x } from './exporter';
export function foo () {
    var x = 1;
}
```

this one should not throw, while this one should throw:

```
import { x } from './exporter';
export function foo () {
    x = 1;
}
```