Adapt `package:messages` to be loading mechanism independent #746

Job	Run time
health / breaking / run breaking health check	53s
health / coverage / run coverage health check	55s
health / changelog / run changelog health check	45s
health / license / run license health check	46s
health / do-not-submit / run do-not-submit health check	45s
health / leaking / run leaking health check	52s
health / comment	4s
	5m 0s

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