Clean up the hphp/system folder

I noticed that directorty structure of hphp/system was a bit scattered, so
I consolidated things to reduce the total number of folders and to put
related things together with each other.

This diff moves the contents of "hphp/system/classes_hhvm" into
"hphp/system", it moves the contents of "hphp/system/lib" into
"hphp/system", moves "hphp/idl" to "hphp/system/idl", and moves the
contents of "hphp/system/globals" into "hphp/system/idl".

hphp/runtime/vm/bytecode.cpp

@@ -60,7 +60,7 @@
 #include "hphp/runtime/base/server/source_root_info.h"
 #include "hphp/runtime/base/util/extended_logger.h"
 
-#include "hphp/system/lib/systemlib.h"
+#include "hphp/system/systemlib.h"
 #include "hphp/runtime/ext/ext_collections.h"
 
 #include "hphp/runtime/vm/name_value_table_wrapper.h"