goinstall: don't wrongly error out with "multiple package names"

Fixes #1215.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/2695042

src/cmd/goinstall/parse.go

@@ -56,7 +56,7 @@ func goFiles(dir string, allowMain bool) (files []string, imports map[string]str
 			// do we return pkgName=="main".
 			// A mix of main and another package reverts
 			// to the original (allowMain=false) behaviour.
-			if allowMain && pkgName == "main" {
+			if s == "main" || pkgName == "main" {
 				return goFiles(dir, false)
 			}
 			return nil, nil, "", os.ErrorString("multiple package names in " + dir)