Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libcmd/installable-derived-path.cc

@@ -0,0 +1,65 @@
+#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 = make_ref<ExtraPathInfo>(),
+    }};
+}
+
+std::optional<StorePath> InstallableDerivedPath::getStorePath()
+{
+    return derivedPath.getBaseStorePath();
+}
+
+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: we accept a symlink chain or an actual
+        // store path.
+        [&](const ExtendedOutputsSpec::Default &) -> DerivedPath {
+            auto storePath = store->followLinksToStorePath(prefix);
+            // Remove this prior to stabilizing the new CLI.
+            if (storePath.isDerivation()) {
+                auto oldDerivedPath = DerivedPath::Built {
+                    .drvPath = makeConstantStorePathRef(storePath),
+                    .outputs = OutputsSpec::All { },
+                };
+                warn(
+                    "The interpretation of store paths arguments ending in `.drv` recently changed. If this command is now failing try again with '%s'",
+                    oldDerivedPath.to_string(*store));
+            };
+            return DerivedPath::Opaque {
+                .path = std::move(storePath),
+            };
+        },
+        // If the user did use ^, we just do exactly what is written.
+        [&](const ExtendedOutputsSpec::Explicit & outputSpec) -> DerivedPath {
+            auto drv = make_ref<SingleDerivedPath>(SingleDerivedPath::parse(*store, prefix));
+            drvRequireExperiment(*drv);
+            return DerivedPath::Built {
+                .drvPath = std::move(drv),
+                .outputs = outputSpec,
+            };
+        },
+    }, extendedOutputsSpec.raw);
+    return InstallableDerivedPath {
+        store,
+        std::move(derivedPath),
+    };
+}
+
+}