nat: make PortMap and PortSet aliases

[thaJeztah]

The PortSet and PortMap types don't carry a real meaning on their own. Their purpose is defined at locations where they're used, such as Container.ExposedPorts or Container.PortBindings.

Their documentation also shows this;

// PortSet is a collection of structs indexed by [PortRangeProto].
// PortMap is a collection of [PortBinding] indexed by [PortRangeProto].

Which, almost literally, describes what a map is. Substituting the types for their implementation doesn't lose any meaning;

// map[PortRangeProto]struct{} is a collection of structs indexed by [PortRangeProto].
// map[PortRangeProto][]PortBinding is a collection of [PortBinding] indexed by [PortRangeProto].

Neither type has any special handling connected to them (no methods or otherwise), so they are just maps with a fancy name attached. Worse, using the extra indirect induces cognitive load; it's not clear from the type that it's "just" a map, indexed by PortBinding, and it's not clear what (kind of) values are in the map.

There are cases where having a type defined can add value to provide a more in-depth description of their intent, but (as shown above) even that is not the case here.

I'm considering these types to be premature abstraction with no good value. As they are a "straight" copy of a map with the same signature, replacing these types preserves backward compatibility; existing code can assign either a PortSet or PortMap, or a map[PortRangeProto].. with the same signature, but we can deprecate the types to give users a nudge to use a regular map instead. The following shows that they are interchangeable;

type config1 struct {
	ExposedPorts container.PortSet
	PortBindings container.PortMap
}
type config2 struct {
	ExposedPorts map[container.PortRangeProto]struct{}
	PortBindings map[container.PortRangeProto][]container.PortBinding
}

var (
	ports         map[container.PortRangeProto]struct{}
	portBindings  map[container.PortRangeProto][]container.PortBinding
	ports2        container.PortSet
	portBindings2 container.PortMap
)
_ = config1{
	ExposedPorts: ports,
	PortBindings: portBindings,
}
_ = config1{
	ExposedPorts: ports2,
	PortBindings: portBindings2,
}
_ = config2{
	ExposedPorts: ports,
	PortBindings: portBindings,
}
_ = config2{
	ExposedPorts: ports2,
	PortBindings: portBindings2,
}

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