Fix in-source builds

The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.

The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.

Fixed maxctrl in-source builds by making the copying of the sources
optional.

server/core/modulecmd.cc

@@ -21,8 +21,8 @@
 #include <maxscale/platform.h>
 #include <maxscale/spinlock.h>
 
-#include "maxscale/filter.h"
-#include "maxscale/monitor.h"
+#include "internal/filter.h"
+#include "internal/monitor.h"
 
 /** Size of the error buffer */
 #define MODULECMD_ERRBUF_SIZE 512