rez lint command

Add a new `rez lint` command.

This command inspects a list of predefined environment variables (`PYTHONPATH`, `LD_LIBRARY_PATH` and `PATH` by default) and reports back information about the entries in the variable.  Information includes:

* duplicates
* paths which do not exist
* paths which exist and are empty

All three of these are potentially considered error conditions resulting in an unclean environment.  This tool can be used as a diagnostic to identify these problems.

Ideally this would also tell you the name-version of the package that introduced the path in question.  Currently this is implied from the path itself.  As discussed, an update to Rex would be required to enable this.