need to detect rez version incompatibities with package.py's

We now have both built-time and runtime features in package.pys that only newer rez versions can consume.

Need to introduce:
- build-time 'requires_rez_version' package attribute;
- runtime 'format_version' package attribute (MAJOR only). This would be an internal serialization number that gets upped when any new runtime features are added to packages. Only rez with format_version >= this will be able to consume these packages.

Start by locating the package.py handling for build-time and runtime package attributes, then trace how package data is serialized and consumed by Rez. Define the compatibility behavior for the two requested version attributes and verify that packages using newer features cannot be consumed by older Rez versions.

Written by the indexing model from the issue text.