fix(treewide): remove unnecessary copying in range for loops

This gets rid of unnecessary copies in range-based-for loops and
local variables, when they are used solely as `const &`.

Also added a fixme comment about a suspicious move out of const,
which might not be intended.

src/libexpr/primops/fetchClosure.cc

@@ -40,7 +40,7 @@ static void runFetchClosureWithRewrite(EvalState & state, const PosIdx pos, Stor
             });
     }
 
-    auto toPath = *toPathMaybe;
+    const auto & toPath = *toPathMaybe;
 
     // check and return