Switch Signals to a map structure

[sdboyer]

Currently, signals are defined in PollyPackage as a list, for the reasons in the comment:

polly/schema/pollypkg.cue

Lines 40 to 50 in c226524

	// NOTE This is a list instead of a struct so that we can allow duplication
	// in the future. That would permit implementations of the "same" signal in
	// different query languages. This would be somewhat analogous to
	// function/method overloading - the consumer's value context can determine
	// whether to use the e.g. promql or flux implementation (assuming both
	// exist).
	//
	// TODO constrain name uniqueness within the list.
	//
	// @doc(metaschema)
	signals?: [Signal, ...Signal]

Addressing list elements in CUE is just...super annoying. I already wrestled with it months ago with scuemata. It may get better, but i don't want to wait, and the justification in the comment is flimsy at best. So, better to use a map structure.