Split OutputsSpec and ExtendedOutputsSpec, use the former more

`DerivedPath::Built` and `DerivationGoal` were previously using a
regular set with the convention that the empty set means all outputs.
But it is easy to forget about this rule when processing those sets.
Using `OutputSpec` forces us to get it right.

src/libexpr/flake/flakeref.cc

@@ -245,7 +245,7 @@ std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragment
     bool isFlake)
 {
     auto [prefix, extendedOutputsSpec] = ExtendedOutputsSpec::parse(url);
-    auto [flakeRef, fragment] = parseFlakeRefWithFragment(prefix, baseDir, allowMissing, isFlake);
+    auto [flakeRef, fragment] = parseFlakeRefWithFragment(std::string { prefix }, baseDir, allowMissing, isFlake);
     return {std::move(flakeRef), fragment, extendedOutputsSpec};
 }