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.cc

@@ -0,0 +1,70 @@
+#include "installable-derived-path.hh"
+#include "derivations.hh"
+
+namespace nix {
+
+std::string InstallableDerivedPath::what() const
+{
+    return derivedPath.to_string(*store);
+}
+
+DerivedPathsWithInfo InstallableDerivedPath::toDerivedPaths()
+{
+    return {{.path = derivedPath, .info = {} }};
+}
+
+std::optional<StorePath> InstallableDerivedPath::getStorePath()
+{
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & bfd) {
+            return bfd.drvPath;
+        },
+        [&](const DerivedPath::Opaque & bo) {
+            return bo.path;
+        },
+    }, derivedPath.raw());
+}
+
+InstallableDerivedPath InstallableDerivedPath::parse(
+    ref<Store> store,
+    std::string_view prefix,
+    ExtendedOutputsSpec extendedOutputsSpec)
+{
+    auto derivedPath = std::visit(overloaded {
+        // If the user did not use ^, we treat the output more liberally.
+        [&](const ExtendedOutputsSpec::Default &) -> DerivedPath {
+            // First, we accept a symlink chain or an actual store path.
+            auto storePath = store->followLinksToStorePath(prefix);
+            // Second, we see if the store path ends in `.drv` to decide what sort
+            // of derived path they want.
+            //
+            // This handling predates the `^` syntax. The `^*` in
+            // `/nix/store/hash-foo.drv^*` unambiguously means "do the
+            // `DerivedPath::Built` case", so plain `/nix/store/hash-foo.drv` could
+            // also unambiguously mean "do the DerivedPath::Opaque` case".
+            //
+            // Issue #7261 tracks reconsidering this `.drv` dispatching.
+            return storePath.isDerivation()
+                ? (DerivedPath) DerivedPath::Built {
+                    .drvPath = std::move(storePath),
+                    .outputs = OutputsSpec::All {},
+                }
+                : (DerivedPath) DerivedPath::Opaque {
+                    .path = std::move(storePath),
+                };
+        },
+        // If the user did use ^, we just do exactly what is written.
+        [&](const ExtendedOutputsSpec::Explicit & outputSpec) -> DerivedPath {
+            return DerivedPath::Built {
+                .drvPath = store->parseStorePath(prefix),
+                .outputs = outputSpec,
+            };
+        },
+    }, extendedOutputsSpec.raw());
+    return InstallableDerivedPath {
+        store,
+        std::move(derivedPath),
+    };
+}
+
+}