toolbox.sh: Add the script path to the module search paths

toolbox.sh: Add the script path to the module search paths

Toolbox requires modules to be located either in ~/.toolbox/include
or in /usr/share/toolbox/include. This is somewhat inconvenient when
developing scripts with modules that are not supposed to be shared
with other scripts.

This commit adds the path of the executing script to the module
search path, allowing script-private modules to be placed in a
directory called "include" next to the script.