Localize require

Make require inside scopedEval to resolve modules againt current module's
filename. This way we can require functions from other modules.

Allows to do `require(...)` calls inside macro bodies:

```
macro sql {
  case { _ $expr:expr } => {
    var parser = require('sql-parser');
    return parser.parse(#{$expr}.token.value);
  } 
}
```

This is very useful when you want to embed some other language inside JS (via ES6 tagged template strings for example), see [sweet-jsx](https://github.com/andreypopp/sweet-jsx/blob/master/example.sjs) or [sweet-sql](https://github.com/andreypopp/sweet-sql/blob/master/examples.sjs) for example.