Move /src to /subprojects

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

subprojects/libutil/file-path.hh

@@ -0,0 +1,47 @@
+#pragma once
+///@file
+
+#include <filesystem>
+
+#include "types.hh"
+#include "os-string.hh"
+
+namespace nix {
+
+/**
+ * Paths are just `std::filesystem::path`s.
+ *
+ * @todo drop `NG` suffix and replace the ones in `types.hh`.
+ */
+typedef std::list<std::filesystem::path> PathsNG;
+typedef std::set<std::filesystem::path> PathSetNG;
+
+/**
+ * Stop gap until `std::filesystem::path_view` from P1030R6 exists in a
+ * future C++ standard.
+ *
+ * @todo drop `NG` suffix and replace the one in `types.hh`.
+ */
+struct PathViewNG : OsStringView
+{
+    using string_view = OsStringView;
+
+    using string_view::string_view;
+
+    PathViewNG(const std::filesystem::path & path)
+        : OsStringView{path.native()}
+    { }
+
+    PathViewNG(const OsString & path)
+        : OsStringView{path}
+    { }
+
+    const string_view & native() const { return *this; }
+    string_view & native() { return *this; }
+};
+
+std::optional<std::filesystem::path> maybePath(PathView path);
+
+std::filesystem::path pathNG(PathView path);
+
+}