False positive warning: unused import when var symbol is used only as assignment target

## Compiler version

3.7.0-RC1
3.7.0-RC2

## Minimized code

Compile with `-Wunused:imports`:

```Scala
object X {
  var global = 0
}

object Main {
  import X.global
  def main(args: Array[String]): Unit = {
    global = 1
  }
}
```


## Output

```scala
unused import
  import X.global
```

## Expectation

The warning should not be there. The code requires the import to compile.
