Extend the CMAKE_MODULE_PATH Special Case

This change extends the logic  in `convert_old_commands` to improve backwards compatibility.

The `CMAKE_MODULE_PATH` variable is `;` separated, however as noted in the [comment in Rez 1](https://github.com/nerdvegas/rez/blob/master/python/rez/rez_config.py#L2056-L2062) it must be surrounded by quotes to make it through the bash commands.  

Although Rez 1 provided logic to translate `:` separated `CMAKE_MODULE_PATH` variables to be `;` separated, we did not (for some reason) use that, and I wonder if others are in the same position.  

As a result nearly all of our packages include:

    - export CMAKE_MODULE_PATH=!ROOT!/cmake';'$CMAKE_MODULE_PATH


Therefore, I'd like to propose a small change to `convert_old_commands` that allows it to replace `';'` with `:` and improve backwards compatibilty.