Don't swallow @utility declarations when @apply is used in nested rules

[philipp-spiess]

Fixes #16935

This PR fixes an issue where the order of how @apply was resolved was incorrect for nested rules. Consider this example:

.rule {
  @apply underline;
  .nested-rule {
    @apply custom-utility;
  }
}

@utility custom-utility {
  @apply flex;
}

The way we topologically sort these, we end up with a list that looks roughly like this:

.rule {
  @apply underline;
  .nested-rule {
    @apply custom-utility;
  }
}

@utility custom-utility {
  @apply flex;
}

.nested-rule {
  @apply custom-utility;
}

As you can see here the nested rule is now part of the top-level list. This is correct because we first have to substitute the @apply inside the @utility custom-utility before we can apply the custom-utility inside .nested-rule. However, because we were using a regular AST walk and because the initial .rule also contains the .nested-rule as child, we would first substitute the @apply inside the .nested-rule, causing the design-system to force resolve (and cache) the wrong value for custom-utility.

Because the list is already flattened, we do not need to recursively look into child declarations when we traverse the sorted list. This PR changes it to use a regular for loop instead of the walk.

Test plan

• Added a regression test
• Rest of tests still green