cmd/go: TestGoGetDashTIssue8181 fails

[josharian]

Broken out from #17472.

$ go test -run=TestGoGetDashTIssue8181 cmd/go
--- FAIL: TestGoGetDashTIssue8181 (8.95s)
	go_test.go:251: running testgo [get -v -t github.com/rsc/go-get-issue-8181/a github.com/rsc/go-get-issue-8181/b]
	go_test.go:270: standard error:
	go_test.go:271: github.com/rsc/go-get-issue-8181 (download)
		Fetching https://golang.org/x/build/cmd/cl?go-get=1
		Parsing meta tags from https://golang.org/x/build/cmd/cl?go-get=1 (status code 200)
		get "golang.org/x/build/cmd/cl": found meta tag main.metaImport{Prefix:"golang.org/x/build", VCS:"git", RepoRoot:"https://go.googlesource.com/build"} at https://golang.org/x/build/cmd/cl?go-get=1
		get "golang.org/x/build/cmd/cl": verifying non-authoritative meta tag
		Fetching https://golang.org/x/build?go-get=1
		Parsing meta tags from https://golang.org/x/build?go-get=1 (status code 200)
		golang.org/x/build (download)
		package golang.org/x/build/cmd/cl: import "golang.org/x/build/cmd/cl" is a program, not an importable package
		
	go_test.go:280: go [get -v -t github.com/rsc/go-get-issue-8181/a github.com/rsc/go-get-issue-8181/b] failed unexpectedly: exit status 1
FAIL
FAIL	cmd/go	10.931s

[0xmohit]

This appears to be failing due to import of main package (the restriction was added for test code in 0f06d0a).

[gopherbot]

CL https://golang.org/cl/32616 mentions this issue.