Factor out InstallableStorePath to its own file, dedup

`nix app` had something called `InstallableDerivedPath` which is
actually the same thing. We go with the later's name because it has
become more correct.

I originally did this change (more hurriedly) as part of #6225 --- a
mini store-only Nix and a full Nix need to share this code. In the first
RFC meeting for https://github.com/NixOS/rfcs/pull/134 we discussed how
some splitting of the massive `installables.cc` could begin prior, as
that is a good thing anyways. (@edolstra's words, not mine!) This would
be one such PR.

src/libcmd/installable-derived-path.hh

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "installables.hh"
+
+namespace nix {
+
+struct InstallableDerivedPath : Installable
+{
+    ref<Store> store;
+    DerivedPath derivedPath;
+
+    InstallableDerivedPath(ref<Store> store, DerivedPath && derivedPath)
+        : store(store), derivedPath(std::move(derivedPath))
+    { }
+
+    std::string what() const override;
+
+    DerivedPathsWithInfo toDerivedPaths() override;
+
+    std::optional<StorePath> getStorePath() override;
+
+    static InstallableDerivedPath parse(
+        ref<Store> store,
+        std::string_view prefix,
+        ExtendedOutputsSpec extendedOutputsSpec);
+};
+
+}