stdenv moveToOutput: move even broken symlinks

I believe this reduces surprises and is actually simpler semantically.
This is important e.g. for relative symlinks when moving both source
and target - now the order of moving won't matter.

Fixes #20723 (a particular instance of the surprise).

pkgs/build-support/setup-hooks/multiple-outputs.sh

@@ -98,7 +98,8 @@ moveToOutput() {
         if [ "${!output}" = "$dstOut" ]; then continue; fi
         local srcPath
         for srcPath in "${!output}"/$patt; do
-            if [ ! -e "$srcPath" ]; then continue; fi
+            # apply to existing files/dirs, *including* broken symlinks
+            if [ ! -e "$srcPath" ] && [ ! -L "$srcPath" ]; then continue; fi
 
             if [ "$dstOut" = REMOVE ]; then
                 echo "Removing $srcPath"