Add ability to use ddtrace_serverless wheels in layer build #559

Job	Run time
build	57s
system-tests / Get parameters / Get parameters	13s
system-tests / Build end-to-end (alb)	1m 11s
system-tests / Build end-to-end (alb-multi)	1m 16s
system-tests / Build end-to-end (apigw-rest)	1m 6s
system-tests / Build end-to-end (apigw-http)	1m 8s
system-tests / Build end-to-end (function-url)	1m 13s
system-tests / parametric	-1s
system-tests / End-to-end #1 / function-url 1	3m 16s
system-tests / End-to-end #1 / alb-multi 1	3m 13s
system-tests / End-to-end #1 / apigw-rest 1	4m 6s
system-tests / End-to-end #1 / alb 1	3m 41s
system-tests / End-to-end #1 / apigw-http 1	4m 3s
system-tests / Report failures	0s
	25m 22s

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