rust-cpython: import utils::files::* function at module level IIRC, it's common in Rust to call functions with the module prefix.

exchange: guard against method invocation on `b2caps=None` args I couldn't figure out how these are called, but the value is pretty obviously set at least for the cases that have tests. Differential Revision: https://phab.mercurial-scm.org/D7512