cmd/go: fix go list -test where C is a dependency.

Currently go list -test runtime/cgo fails with an index out of range
error. This appears to be because the updating of import paths that
happens as part of -test doesn't take into account the fact that the
Internal.Imports of a package do not contain "C", whereas the public
Imports do.

Therefore we skip the public Import of "C" if it exists and continue.

Change-Id: I5cdc8968890fa7e5da3e375718606037d3282754
Reviewed-on: https://go-review.googlesource.com/111175
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -1946,6 +1946,9 @@ func TestGoListTest(t *testing.T) {
 	tg.grepStdout(`^cmd/doc\.test$`, "missing cmd/doc test")
 	tg.grepStdoutNot(`^cmd/dist\.test$`, "unexpected cmd/dist test")
 	tg.grepStdoutNot(`^testing`, "unexpected testing")
+
+	tg.run("list", "-test", "runtime/cgo")
+	tg.grepStdout(`^runtime/cgo$`, "missing runtime/cgo")
 }
 
 // Issue 4096. Validate the output of unsuccessful go install foo/quxx.

src/cmd/go/internal/list/list.go

@@ -318,8 +318,14 @@ func runList(cmd *base.Command, args []string) {
 		}
 		// Update import path lists to use new strings.
 		for _, p := range all {
+			j := 0
 			for i := range p.Imports {
-				p.Imports[i] = p.Internal.Imports[i].ImportPath
+				// Internal skips "C"
+				if p.Imports[i] == "C" {
+					continue
+				}
+				p.Imports[i] = p.Internal.Imports[j].ImportPath
+				j++
 			}
 		}
 		// Recompute deps lists using new strings, from the leaves up.