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/libstore/local-overlay-store.cc

@@ -156,7 +156,7 @@ void LocalOverlayStore::queryGCReferrers(const StorePath & path, StorePathSet &
 StorePathSet LocalOverlayStore::queryValidDerivers(const StorePath & path)
 {
     auto res = LocalStore::queryValidDerivers(path);
-    for (auto p : lowerStore->queryValidDerivers(path))
+    for (const auto & p : lowerStore->queryValidDerivers(path))
         res.insert(p);
     return res;
 }