contrib: tweak import-checker to always use bytes for module names
We could go the other route and make embedded() return this as a
bytes, but then it would be harder to portably doctest, so I prefer
this option.
Differential Revision: https://phab.mercurial-scm.org/D5206
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.