BUG backreferences not working for functions if not imported directly

[lucyleeow]

Creation of mini-galleries with backreferences_dir and doc_module generates an empty rst file if you use a function in an example like this:

import package.module

package.module.my_fun()

It works if you directly import the function in the example:

from package.module import my_fun

my_fun()

or if you import the function in the __init__.py file of the package (i.e.:

from .module import (
    my_fun,
    )

Note that this is only for functions. Classes work fine in all cases.

Refer to sphinx-gallery/sample-project#1 (comment) for suggestion on how to fix.