Add build_commands and private_build_commands functions in package definitions

We should really have the capability to provide build-only environment variables (for example as a way of defining paths to external payloads which need to be carried along with the package at release time).
It would make sense to provide this as a build_commands() / private_build_commands() function to be provided in the package.py for evaluation only at build time. the "if building" method in commands() sort of functions in this way, but formalizing the approach to be used when a package itself is being built would be preferential.

Start in the package.py evaluation path and compare the existing commands() handling of the "if building" condition with the proposed build_commands() and private_build_commands() functions. Trace how build-time environment variables and external payload paths are carried through package building, then define completion as build-only commands being evaluated during builds without affecting normal commands().

Written by the indexing model from the issue text.