Move /src to /subprojects

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

subprojects/libfetchers/registry.hh

@@ -0,0 +1,72 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "fetchers.hh"
+
+namespace nix { class Store; }
+
+namespace nix::fetchers {
+
+struct Registry
+{
+    const Settings & settings;
+
+    enum RegistryType {
+        Flag = 0,
+        User = 1,
+        System = 2,
+        Global = 3,
+        Custom = 4,
+    };
+
+    RegistryType type;
+
+    struct Entry
+    {
+        Input from, to;
+        Attrs extraAttrs;
+        bool exact = false;
+    };
+
+    std::vector<Entry> entries;
+
+    Registry(const Settings & settings, RegistryType type)
+        : settings{settings}
+        , type{type}
+    { }
+
+    static std::shared_ptr<Registry> read(
+        const Settings & settings,
+        const Path & path, RegistryType type);
+
+    void write(const Path & path);
+
+    void add(
+        const Input & from,
+        const Input & to,
+        const Attrs & extraAttrs);
+
+    void remove(const Input & input);
+};
+
+typedef std::vector<std::shared_ptr<Registry>> Registries;
+
+std::shared_ptr<Registry> getUserRegistry(const Settings & settings);
+
+std::shared_ptr<Registry> getCustomRegistry(const Settings & settings, const Path & p);
+
+Path getUserRegistryPath();
+
+Registries getRegistries(const Settings & settings, ref<Store> store);
+
+void overrideRegistry(
+    const Input & from,
+    const Input & to,
+    const Attrs & extraAttrs);
+
+std::pair<Input, Attrs> lookupInRegistries(
+    ref<Store> store,
+    const Input & input);
+
+}