Replace a few bool flags with enums

Functions like copyClosure() had 3 bool arguments, which creates a
severe risk of mixing up arguments.

Also, implement copyClosure() using copyPaths().

src/libexpr/eval.cc

@@ -293,6 +293,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sWrong(symbols.create("wrong"))
     , sStructuredAttrs(symbols.create("__structuredAttrs"))
     , sBuilder(symbols.create("builder"))
+    , repair(NoRepair)
     , store(store)
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)