Proposal: allow top-level conditional for imports

While it is possible to have separate modules using `@if`
```
module foo @if(...);

module foo @if(...);
```

I think it could be neat to also have this supported with `import`, so that less code duplication could occur
```
module foo;
import bar @if(...);
```

As of now, compiler returns `Error: Only '@public' and '@norecurse' are valid attributes here.` as expected.