Remove code directory structure from Plan serialisation format

[bboreham]

example:

{"install.cri":{"github.com/weaveworks/wksctl/pkg/plan/Plan":
	{"install:docker":{"RPM":{"name":"docker-ce","version":"19.03.8"},"meta":{"dependsOn":[]}},

The string "github.com/weaveworks/wksctl/pkg/plan/Plan" is tightly tied to the code structure; I think it would be better to have a more abstract identifier.