Adapt `package:messages` to be loading mechanism independent #753

Job	Run time
health / leaking / run leaking health check	52s
health / coverage / run coverage health check	45s
health / changelog / run changelog health check	45s
health / breaking / run breaking health check	54s
health / license / run license health check	40s
health / do-not-submit / run do-not-submit health check	39s
health / comment	3s
	4m 38s

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