Add ability to use ddtrace_serverless wheels in layer build #546

Job	Run time
build	59s
system-tests / Get parameters / Get parameters	17s
system-tests / Build end-to-end (alb-multi)	1m 12s
system-tests / Build end-to-end (alb)	1m 14s
system-tests / Build end-to-end (apigw-rest)	1m 9s
system-tests / Build end-to-end (apigw-http)	1m 12s
system-tests / Build end-to-end (function-url)	1m 16s
system-tests / parametric	0s
system-tests / End-to-end #1 / apigw-rest 1	3m 50s
system-tests / End-to-end #1 / function-url 1	3m 37s
system-tests / End-to-end #1 / alb 1	3m 53s
system-tests / End-to-end #1 / alb-multi 1	3m 51s
system-tests / End-to-end #1 / apigw-http 1	3m 42s
system-tests / Report failures	-1s
	26m 11s

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