Skip preprocessors for imported files

This would remove support for a buggy feature of including `//= require`s inside `@import`ed .scss files. This wouldn't completely remove `//= require` support. Let me show some examples.

``` scss
// application.scss
//
// still works
//= require layout
//= require users
```

``` scss
// application.scss
//
// works too
@import "layout";
@import "users";
```

``` scss
// application.scss
@import "layout";

// layout.scss
// won't work
//= require "users"
```

Mixing `@import`s and `//= require`s like this has many existing issues. For an example.

``` scss
// application.scss
@import "a";
@import "b";

// users.scss
//= require "c"
```

Would actually produce `["c", "a", "b"]` rather than the expected `["a", "c", "b"]` order.

Targets sass-rails 5.x beta. I'm mostly afraid of applying this to the 4.x branch. I'm not sure if people are depending on this crazyness.

@rafaelfranca would love your :+1: 
In a follow PR, I want to simplify this further. Since we only state explicit support for `.erb`, there would only be two possible values here: An empty `[]` or `[Tilt::ERBTemplate]`.