Move the parser and checks logic into oslo_policy._parser and
oslo_policy._checks respectively. As a consequence, this allows us to
create separate test files for those modules so we now also have
oslo_policy.tests.test_parser and oslo_policy.tests.test_checks. Since
those modules needed some common classes and fixtures it was also
necessary to add oslo_policy.tests.base to service the three test
modules.
Change-Id: I656dcb8fda7b953f5def8ddfaa4d119a8c881965
df5d80759e