Alternative for `register`

[youtux]

We may want to provide an alternative to the register(...) function to inject fixtures in the current module.

I considered having a special variable, like "bdd_fixtures", that the user can declare in the module. We can then check at collection time if a module has this variable and inject the fixtures.

Example:

factoryboy_fixtures = (
    register("foo", FooFactory),
    ...
)

The reason for this is that it's not nice to have to do a from .my_fixtures import * for a user when fixtures are defined across multiple modules and have to be imported in the main conftest.py.