rez-package-pitfalls

The package.py execution model and the errors it produces — why a top-level from x import SomeClass makes the installed package.py unparseable, why module-scope values are frozen on the build machine, why env/this/root look undefined to linters, and why a failed rez-build can leave a broken install behind. Use when a package resolves or builds with a confusing error, when reviewing a package.py for module-scope mistakes, or before releasing one. Covers Rez 3.4.0.