Differentiate CSS import and Sass imports

[xzyfer]

The follow Sass should not error because the @import is a CSS import not a Sass import. This was previously fixed in #1724 but has since regressed.

@mixin foo() {
  @import "https://foo";
}
@include foo;

LibSass 3.3.6

Error: Import directives may not be used within control directives or mixins.
        on line 2 of test.scss
>>   @import "https://foo";
   --^

Ruby Sass 3.4.21

@import "https://foo";

Spec added sass/sass-spec#910