Move modules.h to core/maxscale

Also remove some unnecessary includes + cleanup

server/core/monitor.c

@@ -38,7 +38,6 @@
 #include <maxscale/externcmd.h>
 #include <maxscale/gwdirs.h>
 #include <maxscale/log_manager.h>
-#include <maxscale/modules.h>
 #include <maxscale/mysql_utils.h>
 #include <maxscale/pcre2.h>
 #include <maxscale/secrets.h>
@@ -46,6 +45,7 @@
 
 #include "maxscale/config.h"
 #include "maxscale/monitor.h"
+#include "maxscale/modules.h"
 
 static MXS_MONITOR  *allMonitors = NULL;
 static SPINLOCK monLock = SPINLOCK_INIT;