ensure first/any/one work on nested fields

[klahnakoski]

{"select":{"value":"b.~N~", "aggregate":"one"}} makes sense, so we should allow it.

[klahnakoski]

{
	"from":"unittest",
	"groupby":["task.id","result.group"],
	"select":[
		{"value":"result.ok","aggregate":"min"},
		{"value":"task.tags","aggregate":"one"}
	],
	"where":{"prefix":{"repo.changeset.id":"1e98a73a452d96fae861bd45c1cbef7ae2bd7a91"}}
}