Fix spurious warning at startup under Octave about +pac/+bgp/{get,set}.m

This is a workaround for Octave bug 46849. It complements commit
4c0b2e8c.

The workaround consists in moving the two problematic files to a subfolder
which is not in the path at startup. They will be added to the path later, at a
time when the Octave:shadowed-function warning has already been disabled by
warning_config.m.

Closes: #1817
(cherry picked from commit b60607a5)

matlab/warning_config.m

@@ -49,9 +49,11 @@ if isoctave
     warning('off', 'Octave:classdef-to-struct');
     warning('off', 'Octave:legacy-function'); % For strmatch and isdir
 
-    % The following is necessary because of matlab/+pac/+bgp/{get,set}.m
+    % The following is necessary because of matlab/pac-tools/+pac/+bgp/{get,set}.m
     % which triggers this bug: https://savannah.gnu.org/bugs/?46849
-    % The warning can be reenabled once the bug is fixed.
+    % The warning can be reenabled once the bug is fixed (and the files
+    % mentioned above can then be moved back to matlab/+pac/+bgp/,
+    % see #1817).
     warning('off', 'Octave:shadowed-function');
 else
     % In MATLAB >= 7.7, don't display a warning if we use deprecated