BUG: Can't use imported function if extern function defined (even if not exported)

It gives a weird bug where it says it doesn't find the function. Example:

```py
# math.les
def extern sin(x: float) -> float

export def sin(x: float) -> float
    return sin(x)
```

```py
# main.les
import math

math.sin(1.0)
```

It gives an error: `Function sin not in current scope.`
