Misc Fixes

* Stop the release.data resource being loaded/evaluated twice. This might need a further refactor when acting on the FIXME and TODO comments, but should be enough for the time being.
* Remove unecessary nested imports and fix undeclared variable error.
* Remove reference to REZ_RELEASE_PACKAGES_PATH from RezBuild.cmake - the cmake build system does not provide it and it appears to be unused.