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/context.cc

@@ -132,6 +132,8 @@ static void prim_addDrvOutputDependencies(EvalState & state, const PosIdx pos, V
             },
             [&](const NixStringContextElem::DrvDeep & c) -> NixStringContextElem::DrvDeep {
                 /* Reuse original item because we want this to be idempotent. */
+                /* FIXME: Suspicious move out of const. This is actually a copy, so the comment
+                 above does not make much sense. */
                 return std::move(c);
             },
         }, context.begin()->raw) }),