toolbox: Allow modules to be included from TOOLBOX_HOME

toolbox: Allow modules to be included from TOOLBOX_HOME

The current implementation requires that modules be located in the
global module search path, TOOLBOX_PATH/include. This commit modifies
the implementation so that modules may be included from the user's
TOOLBOX_HOME in addition to the global search path:
When include() is invoked, it will attempt to include the module from
the TOOLBOX_HOME/include first, and only if it didn't succeed, it will
attempt to load the module from TOOLBOX_PATH/include.