Move /src to /subprojects

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

subprojects/libcmd/installable-value.cc

@@ -0,0 +1,68 @@
+#include "installable-value.hh"
+#include "eval-cache.hh"
+#include "fetch-to-store.hh"
+
+namespace nix {
+
+std::vector<ref<eval_cache::AttrCursor>>
+InstallableValue::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>
+InstallableValue::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 UsageError nonValueInstallable(Installable & installable)
+{
+    return UsageError("installable '%s' does not correspond to a Nix language value", installable.what());
+}
+
+InstallableValue & InstallableValue::require(Installable & installable)
+{
+    auto * castedInstallable = dynamic_cast<InstallableValue *>(&installable);
+    if (!castedInstallable)
+        throw nonValueInstallable(installable);
+    return *castedInstallable;
+}
+
+ref<InstallableValue> InstallableValue::require(ref<Installable> installable)
+{
+    auto castedInstallable = installable.dynamic_pointer_cast<InstallableValue>();
+    if (!castedInstallable)
+        throw nonValueInstallable(*installable);
+    return ref { castedInstallable };
+}
+
+std::optional<DerivedPathWithInfo> InstallableValue::trySinglePathToDerivedPaths(Value & v, const PosIdx pos, std::string_view errorCtx)
+{
+    if (v.type() == nPath) {
+        auto storePath = fetchToStore(*state->store, v.path(), FetchMode::Copy);
+        return {{
+            .path = DerivedPath::Opaque {
+                .path = std::move(storePath),
+            },
+            .info = make_ref<ExtraPathInfo>(),
+        }};
+    }
+
+    else if (v.type() == nString) {
+        return {{
+            .path = DerivedPath::fromSingle(
+                state->coerceToSingleDerivedPath(pos, v, errorCtx)),
+            .info = make_ref<ExtraPathInfo>(),
+        }};
+    }
+
+    else return std::nullopt;
+}
+
+}