This moves unisolation into a package key. `cross-build-env: true` means the package
is part of the cross build environment and should be unisolated. `cross-build-files`
gives a list of files that should be copied from the built copy of the package into the host
copy of the package.
This will allow us to construct a cross build environment automatically as part of building
packages. If we have these files and the Python include directory, this is sufficient for
cross-building binary packages.
Gyeongjae Choi