rustpkg: Do not guess version if not given

rustpkg accessed git repo to read tags and guess package version,
which is not quite useful since it can be given explicitly from command
line.

This also fixes output of `rustpkg prefer`.
extra::workcache: Remove unused Logger
rustpkg: Recover `-h`/`--help` option
rustpkg::version: Remove unused variants/impls
rustpkg::version: Replace Version by Option<~str>
fixup: guess version
fixup: guess version