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/query_classifier.cc

@@ -11,16 +11,16 @@
  * Public License.
  */
 
-#include "maxscale/query_classifier.h"
+#include "internal/query_classifier.h"
 #include <maxscale/log_manager.h>
 #include <maxscale/modutil.h>
 #include <maxscale/alloc.h>
 #include <maxscale/platform.h>
 #include <maxscale/pcre2.h>
 #include <maxscale/utils.h>
-#include "maxscale/trxboundaryparser.hh"
 
-#include "../core/maxscale/modules.h"
+#include "internal/modules.h"
+#include "internal/trxboundaryparser.hh"
 
 //#define QC_TRACE_ENABLED
 #undef QC_TRACE_ENABLED