feat(core): support for using SPM (Swift Package Manager) #28

Job	Run time
web (tests)	6m 26s
web (tests)	6m 26s
web (packages/cloud_firestore/cloud_firestore/example)	5m 52s
web (packages/cloud_firestore/cloud_firestore/example)	5m 52s
web-app-check	5m 38s
web-app-check	5m 38s
web-wasm (tests)	4m 0s
web-wasm (tests)	4m 0s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	15m 17s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	6m 33s
	1h 5m 42s

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