tests/test-gendoc-zh_TW.t

rust: Remove the `rustext.parsers` module

It only exported Rust implementations of the parse_dirstate and pack_dirtate
functions, which are only used (anymore) when Rust is not enabled.

fakedirstatewritetime.py was detecting the presence of `rustext.parsers`
but what it really wants to know is whether the Rust implementation of
`dirstatemap` is used. This changes it to detect `rustext.dirstate` instead.

Differential Revision: https://phab.mercurial-scm.org/D11459

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors