cmd/go: panic in work.WriteCoverMetaFilesFile for "go test -n -cover" in root src dir #67953
Description
On tip, I accidentally ran this command below in the root source directory (instead of a subdir).
$ cd `go env GOROOT`
$ cd src
$ go test -n -coverpkg=internal/coverage/decodecounter ./...
This should succeed, but instead I got this crash:
...
/w/go.master/pkg/tool/linux_amd64/compile -o $WORK/b2278/_pkg_.a -trimpath "$WORK/b2278=>" -p time/tzdata -std -complete -buildid kQeKaOUArGGmE6IQy2SI/kQeKaOUArGGmE6IQy2SI -c=4 -nolocalimports -importcfg $WORK/b2278/importcfg -pack ./time/tzdata/tzdata.go ./time/tzdata/zzipdata.go
/w/go.master/pkg/tool/linux_amd64/buildid -w $WORK/b2278/_pkg_.a # internal
panic: unexpected mode built-in package
goroutine 2504 [running]:
cmd/go/internal/work.WriteCoverMetaFilesFile(0xc001d92330?, {0x2f?, 0x0?}, 0xc000d0ef20)
/w/go.master/src/cmd/go/internal/work/cover.go:116 +0x4a7
cmd/go/internal/work.ActorFunc.Act(0xc17ab8?, 0xfefb20?, {0xc17ab8?, 0xfefb20?}, 0xc00292df10?)
/w/go.master/src/cmd/go/internal/work/action.go:76 +0x2d
cmd/go/internal/work.(*Builder).Do.func3({0xc17ab8, 0xfefb20}, 0xc000d0ef20)
/w/go.master/src/cmd/go/internal/work/exec.go:152 +0x7a4
cmd/go/internal/work.(*Builder).Do.func4()
/w/go.master/src/cmd/go/internal/work/exec.go:221 +0xae
created by cmd/go/internal/work.(*Builder).Do in goroutine 1
/w/go.master/src/cmd/go/internal/work/exec.go:207 +0x3d1
What appears to be happening is that work.WriteCoverMetaFilesFile isn't properly handling dummy/builtin packages such as "unsafe" in this particular code path.