Move /src to /subprojects

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

subprojects/libcmd/network-proxy.hh

@@ -0,0 +1,22 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Environment variables relating to network proxying. These are used by
+ * a few misc commands.
+ *
+ * See the Environment section of https://curl.se/docs/manpage.html for details.
+ */
+extern const StringSet networkProxyVariables;
+
+/**
+ * Heuristically check if there is a proxy connection by checking for defined
+ * proxy variables.
+ */
+bool haveNetworkProxyConnection();
+
+}