Support selecting API groups to build

[kazk]

Just an idea for Arnavion/k8s-openapi#77. Finding the dependencies between modules should be possible because each package has imports (FileDescriptorProto has dependency field).

k8s-pb/k8s-pb-codegen/protos/api/core/v1/generated.proto

Lines 22 to 28 in 7b4bf26

	package api.core.v1;
	import "apimachinery/pkg/api/resource/generated.proto";
	import "apimachinery/pkg/apis/meta/v1/generated.proto";
	import "apimachinery/pkg/runtime/generated.proto";
	import "apimachinery/pkg/runtime/schema/generated.proto";
	import "apimachinery/pkg/util/intstr/generated.proto";

We also don't need to worry about multiplying that by version features if we version this using Kubernetes version (#10).