cmd/go: fix TestGoGetDashTIssue8181

The test case was importing golang.org/x/build/cmd/cl,
which is a package main and cannot be imported.
The test case (stored in a separate repo) has been changed
to import golang.org/x/build/gerrit. Update the test accordingly.

Fixes #17702.

Change-Id: I80e150092111b5a04bb00c992b32edb271edb086
Reviewed-on: https://go-review.googlesource.com/32616
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -1913,7 +1913,7 @@ func TestGoGetDashTIssue8181(t *testing.T) {
 	tg.setenv("GOPATH", tg.path("."))
 	tg.run("get", "-v", "-t", "github.com/rsc/go-get-issue-8181/a", "github.com/rsc/go-get-issue-8181/b")
 	tg.run("list", "...")
-	tg.grepStdout("x/build/cmd/cl", "missing expected x/build/cmd/cl")
+	tg.grepStdout("x/build/gerrit", "missing expected x/build/gerrit")
 }
 
 func TestIssue11307(t *testing.T) {