If the python package matching trytond is not installed alongside to trytond,
PYTHONPATH is not set (or pointing to a wrong version) and trytond (and the
trytond tools) would not find the trytond modules.
This change introduces a new native-search-path GUIX_TRYTOND_MODULES_PATH.
Trytond gets patched to honor this environment variable.
Thanks to Maxim Cournoyer, whose code for guix-pythonpath-search-path
was the template for guix-trytonpath-search-path.
* gnu/packages/patches/trytond-add-guix_trytond_path.patch: New file.
* gnu/local.mk: (dist_patch_DATA): Add it.
* gnu/packages/tryton.scm (trytond): Use it.
[native-search-paths]: New element.
(guix-trytonpath-search-path): New function.
