Don't call SchedulablePodGroups when only 1 node group gets created

[morshielt]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR introduces an optimization to scale-up process by skipping the expensive SchedulablePodGroups simulation when only 1 node group is created, as the result is only used by balanceScaleUps (which doesn't matter when we get 1 node group created).

Instead of the expensive call, we're now only patching the node group ID directly (also fixed the stale/duplicate podEquivalenceGroups.SchedulableGroups entries) for reporting correctness.

To keep the schedulablePodGroups (which now won't be recalculated in 1 node group cases) in a more contained scope and prevent future uses depending on the now skipped calculations, the PR includes a refactor that extracted the scale-up planning logic into a separate planScaleUp method.

Which issue(s) this PR fixes:

n/a

Special notes for your reviewer:

Commits are split into logical parts, 1st only moving the code without meaningful changes. PTAL

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[k8s-ci-robot]

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This pull-request has been approved by: morshielt
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[morshielt]

/assign x13n