covermerger: parallelize and use monorepo #11889

Job	Run time
dashboard	2m 0s
aux	3m 52s
build	2m 39s
old	4m 19s
arch (presubmit_arch_linux)	7m 54s
arch (presubmit_arch_freebsd)	4m 36s
race	6m 51s
race_dashboard	2m 35s
arch (presubmit_arch_netbsd)	3m 42s
arch (presubmit_arch_openbsd)	3m 34s
arch (presubmit_arch_darwin)	3m 30s
arch (presubmit_arch_windows)	2m 37s
arch (presubmit_arch_executor)	2m 53s
	51m 2s

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