fetchurl: add passthru resolvedUrl (#398843)

pkgs/build-support/fetchurl/default.nix

···
       }
     else
       throw "fetchurl requires a hash for fixed-output derivation: ${lib.generators.toPretty urls_}";
+
+  resolvedUrl =
+    let
+      mirrorSplit = lib.match "mirror://([[:alpha:]]+)/(.+)" url;
+      mirrorName = lib.head mirrorSplit;
+      mirrorList =
+        if lib.hasAttr mirrorName mirrors then
+          mirrors."${mirrorName}"
+        else
+          throw "unknown mirror:// site ${mirrorName}";
+    in
+    if mirrorName == null then url else "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";
 in
 
 assert
···
 
     inherit meta;
     passthru = {
-      inherit url;
+      inherit url resolvedUrl;
     }
     // passthru;
   }