Move /src to /subprojects

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

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

@@ -0,0 +1,30 @@
+#include "binary-cache-store.hh"
+
+namespace nix {
+
+struct HttpBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
+{
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+
+    HttpBinaryCacheStoreConfig(std::string_view scheme, std::string_view _cacheUri, const Params & params);
+
+    Path cacheUri;
+
+    const std::string name() override
+    {
+        return "HTTP Binary Cache Store";
+    }
+
+    static std::set<std::string> uriSchemes()
+    {
+        static bool forceHttp = getEnv("_NIX_FORCE_HTTP") == "1";
+        auto ret = std::set<std::string>({"http", "https"});
+        if (forceHttp)
+            ret.insert("file");
+        return ret;
+    }
+
+    std::string doc() override;
+};
+
+}