Don't rewrite source maps for @layer rules

[thecrypticace]

Since we generate utilities on demand the source maps point back to the corresponding @tailwind directive for that rule. For example, the component .container points back to the @tailwind components at-rule.

We would do this unconditionally for all generated output by Tailwind CSS. Since @layer components and @layer utilities in CSS register custom utilities, the source maps for these could be lost when the files containing the @layer and @tailwind directives were different. This happens when using postcss-import for example:

/* Source is: "node_modules/tailwindcss/components.css" */
@import "tailwindcss/components";

/* Source is: "input.css" */
@layer components {
  .test {
    color: red;
  }
}

Because of this we would throw away the original source information and it appeared, to later tools, that the original source map was just @tailwind components from the components.css file instead of having sources from components.css AND from input.css. This in turn broke Vite's url-rewriting because it relies on the original source map input.

Curiously, with postcss-import, this problem doesn't appear for local CSS file imports. Only imports coming from node_modules (unless I did something wrong 🙃 ). I would have expected it to rewrite the rules from inside the CSS file to point to the @import but perhaps it treats node_modules stuff differently.

In any case, this PR fixes the source maps so that rules generated from @layer point back to the original source.

Fixes #8966