[Module System] Load CSS from modules used by imported files

Closes #2666

accepted/module-system.changes.md

@@ -11,6 +11,8 @@
 
 * Add a `$module` parameter to `get-function()`.
 
+* Load CSS from modules used by imported files.
+
 * Clarify the behavior of the first law of extend when multiple modules extend
   the same selector.
 

accepted/module-system.md

@@ -1442,17 +1442,23 @@ context](#import-context) `import`, and a mutable [module](#module) `module`.
 * If `file` is currently being executed, throw an error.
 
 * Let `imported` be the result of [executing](#executing-files) `file` with the
-  empty configuration and `import` as its import context, with the following
-  differences:
-
-  * If the `@import` rule is nested within at-rules and/or style rules, that
-    context is preserved when executing `file`.
-
-  * The generated CSS for style rules or at-rules in `file` is appended to the
-    `module`'s CSS.
+  empty configuration and `import` as its import context, except that if the
+  `@import` rule is nested within at-rules and/or style rules, that context is
+  preserved when executing `file`.
 
   > Note that this execution can mutate `import`.
 
+* Let `css` be the result of [resolving extensions](#resolving-extensions) for
+  `imported`, except that if the `@import` rule is nested within at-rules and/or
+  style rules, that context is added to CSS that comes from modules loaded by
+  `imported`.
+
+  > This creates an entirely separate CSS tree with an entirely separate
+  > `@extend` context than normal `@use`s of these modules. This means their CSS
+  > may be duplicated, and they may be extended differently.
+
+* Add `css` to `module`'s CSS.
+
 * Add `imported`'s [extensions](#extension) to `module`.
 
 * For each member `member` in `imported`: