discovery: 'all-modules' returns modules in path order.

discovery: 'all-modules' returns modules in path order.

A particular effect of this is that if there are ambiguous
packages in a directory specified with `-L module_dir` and the
distribution, the version from `module_dir` will be loaded,
which is usually what would be expected. (E.g. for `guix build`
or `guix package -i`.)

* guix/discovery.scm (all-modules): Return modules in path order.
* tests/guix-package.sh: Test local definitions take precedence.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>