simplify and refactor runNextSchedulableTask

[pritidesai]

runNextSchedulableTask is complex and has too many responsibilities, including:

• deciding what to execute next
• resolving references
• actually executing

From @bobcatfish "those feel like at least 3 distinct responsibilities. i think if we get a chance it would be great to detangle this a bit."

Refactor this single function and add unit test.

/kind cleanup

[pritidesai]

/assign

[tekton-robot]

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[pritidesai]

Haven't been able to simplify this yet, its on my todo list.
/remove-lifecycle stale

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[tekton-robot]

Stale issues rot after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle rotten with a justification.
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/lifecycle rotten

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