Various Build-related issues [\548](https://github.com/AcademySoftwareFoundation/rez/pull/548)
Addressed Issues
* [433](https://github.com/AcademySoftwareFoundation/rez/issues/433): "package_definition_build_python_paths" defined
paths are not available from top level in package.py
* [442](https://github.com/AcademySoftwareFoundation/rez/issues/442): "rez-depends" and "private_build_requires"
* [416](https://github.com/AcademySoftwareFoundation/rez/issues/416): Need currently-building-variant build variables
* [547](https://github.com/AcademySoftwareFoundation/rez/issues/547): rez-cp follows symlinks within package payload
Notes
The biggest update in this release is the introduction of new variables accessible at early-bind time:
building, build_variant_index and build_variant_requires. This allows you to do things like define
different private_build_requires per-variant, or a requires that is different at runtime than it is
at build time. In order to get this to work, a package.py is now re-evaluated multiple times when a
build occurs - once pre-build (where 'building' is set to False), and once per variant build. Please
see the updated wiki for more details: https://github.com/AcademySoftwareFoundation/rez/wiki/Package-Definition-Guide#available-objects
A new build-time env-var, REZ_BUILD_VARIANT_REQUIRES, has been added. This mirrors the new
build_variant_requires var mentioned above.
rez-depends has been updated to only include the private_build_requires of the package being queried
(previously, all packages' private build reqs were included, which is not useful). Recall that the
previous release fixes the issue where private_build_requires was being stripped from released
packages.
The entirety of a package definition file can now see the extra build-time modules available via the
package_definition_build_python_paths config setting. Previously, only early bound functions could
see these.
There was an issue with package copying (and thus the rez-cp tool) where symlinks within a package's
payload were expanded out to their source files at copy time. The default now is to keep such symlinks
intact - but hte previous behavior can still be accessed with the rez-cp --follow-symlinks option.