Convert Old Commands Fix and Import Cleanup

Couple of changes here:

* Add tests for `convert_old_commands` function (not comprehensive).
* Updated `convert_old_commands` to cater for unhandled case similar to `export A=$A:foo`.
* Fixed typos in `src/rez/vendor/version/version.py` and `src/rez/cli/bind.py`.  These were generating unhandled exceptions.
* Fixed incorrect import in `src/rez/cli/build.py`.  This was generating unhandled exceptions.
* Adding missing imports in `src/rez/packages.py`, `src/rez/dot.py` and `src/rezplugins/shell/windows.py`.  This was generating unhandled exceptions.
* Removed unused imports in all other files.

Sorry, the imports were getting annoying so I went through and cleaned them up based on what Eclipse was telling me.  Hope that's ok.

All tests pass.