Consolidate private header files

Private functions were previously declared

- in header files in the root directory
- in public headers guarded with IN_LIBXML
- in libxml.h
- redundantly in source files that used them.

Consolidate all private header files in include/private.

debugXML.c

@@ -30,6 +30,8 @@
 #include <libxml/relaxng.h>
 #endif
 
+#include "private/error.h"
+
 #define DUMP_TEXT_TYPE 1
 
 typedef struct _xmlDebugCtxt xmlDebugCtxt;