Implement the expand-id API.

How about this test case too?

{
	about: "fully qualified URL that does not exist",
	url:   "trusty/wordpress-99",
	expect: []params.ExpandedId{
		{Id: "cs:utopic/wordpress-42"},
		{Id: "cs:trusty/wordpress-47"},
		{Id: "cs:bundle/wordpress-0"},
	},
}