Support distinct triplets & package sets for host and target builds

[jherico]

When building for platforms like Android, you may want some vcpkg packages built for the host system to provide build toolchain functionality (for instance, spirv-tools), and other packages built for the target environment (so you can link them against your actual binary).

This should be implemented by supporting the TRIPLET and HOST_TRIPLET as well as HOST_PACKAGES in addition to the existing PACKAGES.