+ Add check for reference to environment variable that should be replaced with rex.
+ Fix unused imports, typo, and missing imports.
+ Remove incorrect use of os.pathsep.
+ Extend regex.
Merge remote-tracking branch 'mark/convert_old_commands_fix' into 2.0

Conflicts:
	src/rez/_sys/_setup.py
	src/rez/build_process.py
	src/rez/package_resources.py
	src/rez/packages.py
	src/rez/resolved_context.py
	src/rez/solver.py
	src/rez/status.py
	src/rez/tests/test_rex.py
	src/rez/vendor/version/version.py
	src/rezplugins/release_vcs/stub.py