This adds two packages:
mu/pkg/compiler/clouds
mu/pkg/compiler/schedulers
And introduces enums for the cloud targets we expect to support.
It also adds the ability at the command line to specify a provider;
for example:
$ mu build --target=aws # AWS native
$ mu build --target=aws:ecs # AWS ECS
$ mu build -t=gcp:kubernetes # Kube on GCP