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/command-installable-value.hh

@@ -0,0 +1,13 @@
+#include "installable-value.hh"
+#include "command.hh"
+
+namespace nix {
+
+struct InstallableValueCommand : InstallableCommand
+{
+    virtual void run(ref<Store> store, ref<InstallableValue> installable) = 0;
+
+    void run(ref<Store> store, ref<Installable> installable) override;
+};
+
+}