Flatten FlavorResourceQuantities

[gabesaba]

What would you like to be cleaned:
FlavorResourceQuantities, a map[string]map[string]int64, is ubiquitous in the codebase. We have a lot of code that processes it in a nested loop, often allocating the inner map in the middle. It started getting in my way during the implementation of #79, and I started using a flat version of this map, resulting in significantly simpler code.

Unfortunately, this type is everywhere and the refactor is quite involved. I propose the following steps:

• Move FlavorResourceQuantities to its own package
• Define new types, a FlavorResource tuple, and FlavorResourceQuantitiesFlat
• Update tests without changing functionality - we will define test data using the new type, but convert it back to the old type with a helper function
• Update non-test source to use this new type
• Cleanup; rename FlavorResourceQuantitiesFlat to FlavorResourceQuantities

The first three parts are safe. The 4th part has more risk, and can be done alongside the implementation of #79.

Why is this needed:
This old type is a mismatch for the problem, and is resulting in unnecessarily nested/complex code.

To help motivate this change, and see why I think the type is a better fit for the problem, consider the following refactors which were possible:

this was reduced to a single for-loop without branching

kueue/pkg/cache/clusterqueue.go

Lines 520 to 532 in 53a0341

	func updateFlavorUsage(wi *workload.Info, flvUsage FlavorResourceQuantities, m int64) {
	for _, ps := range wi.TotalRequests {
	for wlRes, wlResFlv := range ps.Flavors {
	v, wlResExist := ps.Requests[wlRes]
	flv, flvExist := flvUsage[wlResFlv]
	if flvExist && wlResExist {
	if _, exists := flv[wlRes]; exists {
	flv[wlRes] += v * m
	}
	}
	}
	}
	}

this was reduced to a single for-loop, with only 1 more level of branching (the if statements)

kueue/pkg/cache/clusterqueue.go

Lines 534 to 555 in 53a0341

	func updateCohortUsage(wi *workload.Info, cq *ClusterQueue, m int64) {
	for _, ps := range wi.TotalRequests {
	for wlRes, wlResFlv := range ps.Flavors {
	v, wlResExist := ps.Requests[wlRes]
	flv, flvExist := cq.Cohort.Usage[wlResFlv]
	if flvExist && wlResExist {
	if _, exists := flv[wlRes]; exists {
	after := cq.Usage[wlResFlv][wlRes] - cq.guaranteedQuota(wlResFlv, wlRes)
	// rollback update cq.Usage
	before := after - v*m
	if before > 0 {
	flv[wlRes] -= before
	}
	// simulate updating cq.Usage
	if after > 0 {
	flv[wlRes] += after
	}
	}
	}
	}
	}
	}

/assign