ENH: lazy_xp_function namespaces support

[crusaderky]

Working in scipy I realized that forcing the developer who writes the tests to import <names> from <module> can be extremely cumbersome - see for example scipy.stats.tests.test_stats.

This PR allows simply calling

import <module>
lazy_xp_modules = [<module>]

def test_<funcname>(xp):
    <module>.<funcname>(...)

[crusaderky]

@lucascolley ready for review and merge

[lucascolley]

thanks @crusaderky, LGTM!!