No global settings in libnixfetchers and libnixflake

Progress on #5638

There are still a global fetcher and eval settings, but they are pushed
down into `libnixcmd`, which is a lot less bad a place for this sort of
thing.

Continuing process pioneered in
52bfccf8d8.

src/libflake/flake/settings.cc

@@ -0,0 +1,7 @@
+#include "flake/settings.hh"
+
+namespace nix::flake {
+
+Settings::Settings() {}
+
+}