Move /src to /subprojects

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

subprojects/libutil/exec.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+#include "os-string.hh"
+
+namespace nix {
+
+/**
+ * `execvpe` is a GNU extension, so we need to implement it for other POSIX
+ * platforms.
+ *
+ * We use our own implementation unconditionally for consistency.
+ */
+int execvpe(const OsChar * file0, const OsChar * const argv[], const OsChar * const envp[]);
+
+}