fetcher: Move high level functions into "fetcher-util"

Conceptually these now lay on top of the core API, and don't reference libsoup.
This is preparation for libcurl porting, but it's also just generally better.

Closes: #636
Approved by: jlebon

src/libostree/ostree-metalink.c

@@ -21,6 +21,7 @@
 #include "config.h"
 
 #include "ostree-metalink.h"
+#include "ostree-fetcher-util.h"
 #include <gio/gfiledescriptorbased.h>
 
 #include "otutil.h"