refactor: Split input validation into separate steps #226

Job	Run time
cli-integration (macos)	14s
updater-test-args / validate-inputs	2s
updater-create-pr / validate-inputs	4s
updater-test-args / cancel-previous-run	3s
updater-create-pr / cancel-previous-run	4s
cli-integration (windows)	21s
cli-integration (ubuntu)	15s
updater-test-args / update	0s
updater-create-pr / update	0s
updater-test-outputs	0s
	1m 3s

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