bind: handle import paths without trailing slashes correctly

If ever the gobind tool switches to a source importer (see CL
99777), make sure import paths are without trailing slashes are
handled correctly.

Change-Id: Ib3ce15f8dcd53dce09a5bb183e4013a5deba39b4
Reviewed-on: https://go-review.googlesource.com/99776
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/types.go

@@ -161,7 +161,7 @@ func pkgFirstElem(p *types.Package) string {
 	path := p.Path()
 	idx := strings.Index(path, "/")
 	if idx == -1 {
-		return ""
+		return path
 	}
 	return path[:idx]
 }