Building a cross-compiled gprbuild-boot would obviously not succeed.
This change is kind of academic since we don't really have a working Ada
cross-compiler in nixpkgs, but helps verifying changes to the
gnatPackages infrastructure.
Ada depencencies musst be build with the same gnat version as the
project. Use a namespace as preperation to build with different gnat
versions.
gprbuild and gnatprove are still globaly visable.
