Move /src to /subprojects

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

subprojects/libflake/flake/settings.hh

@@ -0,0 +1,50 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "config.hh"
+#include "util.hh"
+
+#include <map>
+#include <limits>
+
+#include <sys/types.h>
+
+namespace nix::flake {
+
+struct Settings : public Config
+{
+    Settings();
+
+    Setting<bool> useRegistries{
+        this,
+        true,
+        "use-registries",
+        "Whether to use flake registries to resolve flake references.",
+        {},
+        true,
+        Xp::Flakes};
+
+    Setting<bool> acceptFlakeConfig{
+        this,
+        false,
+        "accept-flake-config",
+        "Whether to accept nix configuration from a flake without prompting.",
+        {},
+        true,
+        Xp::Flakes};
+
+    Setting<std::string> commitLockFileSummary{
+        this,
+        "",
+        "commit-lock-file-summary",
+        R"(
+          The commit summary to use when committing changed flake lock files. If
+          empty, the summary is generated based on the action performed.
+        )",
+        {"commit-lockfile-summary"},
+        true,
+        Xp::Flakes};
+};
+
+}