Add ability to use ddtrace_serverless wheels in layer build #560

Job	Run time
build	1m 6s
system-tests / Get parameters / Get parameters	14s
system-tests / Build end-to-end (apigw-http)	1m 10s
system-tests / Build end-to-end (alb-multi)	1m 18s
system-tests / Build end-to-end (alb)	1m 10s
system-tests / Build end-to-end (function-url)	1m 30s
system-tests / Build end-to-end (apigw-rest)	1m 13s
system-tests / parametric	0s
system-tests / End-to-end #1 / alb-multi 1	3m 10s
system-tests / End-to-end #1 / alb 1	4m 4s
system-tests / End-to-end #1 / apigw-http 1	3m 23s
system-tests / End-to-end #1 / apigw-rest 1	3m 43s
system-tests / End-to-end #1 / function-url 1	3m 44s
system-tests / Report failures	0s
	25m 45s

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