nix shell: Handle output paths that are symlinks

This requires moving resolveSymlinks() into SourceAccessor. Also, it
requires LocalStoreAccessor::maybeLstat() to work on parents of the
store (to avoid an error like "/nix is not in the store").

Fixes #10375.

tests/functional/shell-hello.nix

@@ -1,6 +1,6 @@
 with import ./config.nix;
 
-{
+rec {
   hello = mkDerivation {
     name = "hello";
     outputs = [ "out" "dev" ];
@@ -24,6 +24,14 @@ with import ./config.nix;
       '';
   };
 
+  hello-symlink = mkDerivation {
+    name = "hello-symlink";
+    buildCommand =
+      ''
+        ln -s ${hello} $out
+      '';
+  };
+
   salve-mundi = mkDerivation {
     name = "salve-mundi";
     outputs = [ "out" ];