pkg/cover: fix handling of compile unit name #16802

Job	Run time
race	7m 42s
race_dashboard	3m 19s
build	5m 29s
aux	4m 14s
dashboard	2m 47s
arch (presubmit_arch_linux)	10m 16s
arch (presubmit_arch_netbsd)	4m 1s
arch (presubmit_arch_freebsd)	6m 14s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_darwin)	3m 45s
gvisor	3m 55s
arch (presubmit_arch_windows)	3m 40s
arch (presubmit_arch_executor)	3m 28s
old	6m 48s
	1h 9m 23s

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