RELEASE 2.5.0: package.py: Preprocessing, Shared Code, Function Attributes, Requirements Expansion

This release adds a number of co-related features:

• Package preprocessing (see https://github.com/nerdvegas/rez/wiki/Package-Definition-Guide#package-preprocessing);
• Shared code in package.py files (see https://github.com/nerdvegas/rez/wiki/Package-Definition-Guide#sharing-code-across-package-definition-files);
• Package attributes as early-/late- binding functions (see https://github.com/nerdvegas/rez/wiki/Package-Definition-Guide#package-attributes-as-functions).
• Requirements expansion (see https://github.com/nerdvegas/rez/wiki/Package-Definition-Guide#requirements-expansion)

Enjoy.
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