rust-cpython: do not convert warning pattern to utf-8 bytes
On Unix, both Rust Path and Mercurial expect a locale-dependent bytes,
and we don't support Windows yet.
Differential Revision: https://phab.mercurial-scm.org/D7614
rust-cpython: import utils::files::* function at module level
IIRC, it's common in Rust to call functions with the module prefix.
Differential Revision: https://phab.mercurial-scm.org/D7613