Zcu: fix @embedFile of "std", "root", "builtin"

Author: mlugg

src/Zcu/PerThread.zig

@@ -2382,7 +2382,16 @@ pub fn embedFile(
     const zcu = pt.zcu;
     const gpa = zcu.gpa;
 
-    if (cur_file.mod.?.deps.get(import_string)) |mod| {
+    const opt_mod: ?*Module = m: {
+        if (mem.eql(u8, import_string, "std")) break :m zcu.std_mod;
+        if (mem.eql(u8, import_string, "root")) break :m zcu.root_mod;
+        if (mem.eql(u8, import_string, "builtin")) {
+            const opts = cur_file.mod.?.getBuiltinOptions(zcu.comp.config);
+            break :m zcu.builtin_modules.get(opts.hash()).?;
+        }
+        break :m cur_file.mod.?.deps.get(import_string);
+    };
+    if (opt_mod) |mod| {
         const resolved_path = try std.fs.path.resolve(gpa, &.{
             zcu.comp.cwd,
             mod.root.root_dir.path orelse ".",