Support @:import meta-data for extern python modules.

This adds support for the `@:import` metadata for extern classes to be used instead of `__init__` methods like this:

```haxe
@:import("flask")
extern class FlaskModule {} // generates "import flask as FlaskModule"

@:import("flask", "Flask") // generates "from flask import Flask as FlaskObject"
extern class FlaskObject {}
```

@Simn, @frabbit, what do you think?