Move value-only methods to InstallableValue

These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.

Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.

Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.

src/libcmd/installables.cc

@@ -364,23 +364,6 @@ DerivedPathWithInfo Installable::toDerivedPath()
     return std::move(buildables[0]);
 }
 
-std::vector<ref<eval_cache::AttrCursor>>
-Installable::getCursors(EvalState & state)
-{
-    auto evalCache =
-        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
-            [&]() { return toValue(state).first; });
-    return {evalCache->getRoot()};
-}
-
-ref<eval_cache::AttrCursor>
-Installable::getCursor(EvalState & state)
-{
-    /* Although getCursors should return at least one element, in case it doesn't,
-       bound check to avoid an undefined behavior for vector[0] */
-    return getCursors(state).at(0);
-}
-
 static StorePath getDeriver(
     ref<Store> store,
     const Installable & i,