Move /src to /subprojects

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

subprojects/libstore/local-binary-cache-store.hh

@@ -0,0 +1,23 @@
+#include "binary-cache-store.hh"
+
+namespace nix {
+
+struct LocalBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
+{
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+
+    LocalBinaryCacheStoreConfig(std::string_view scheme, PathView binaryCacheDir, const Params & params);
+
+    Path binaryCacheDir;
+
+    const std::string name() override
+    {
+        return "Local Binary Cache Store";
+    }
+
+    static std::set<std::string> uriSchemes();
+
+    std::string doc() override;
+};
+
+}