Manifest methods should return maps instead of slices

[sdboyer]

type Manifest interface {
    DependencyConstraints() []ProjectConstraint
    TestDependencyConstraints() []ProjectConstraint
}

This is dumb.

There is no case in which it makes sense to return more than one ProjectConstraint for a given ProjectRoot. But having the method return a slice allows exactly that. These methods should be changed to return map[ProjectRoot]ProjectProperties - the same as overrides. There are still fugly ambiguities than can occur with a map, but fewer than with a slice.