Adapt `package:messages` to be loading mechanism independent #755

Job	Run time
health / license / run license health check	43s
health / changelog / run changelog health check	54s
health / breaking / run breaking health check	44s
health / coverage / run coverage health check	49s
health / do-not-submit / run do-not-submit health check	44s
health / leaking / run leaking health check	56s
health / comment	3s
	4m 53s

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