gosrc: fix IsDir implementation

Fixes golang/gddo#504

Change-Id: Ic17c2b10690cb156678759d1e5bb1632828a34e2
Reviewed-on: https://go-review.googlesource.com/59191
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
diff --git a/gosrc/build.go b/gosrc/build.go
index 5ffc969..98c8c0b 100644
--- a/gosrc/build.go
+++ b/gosrc/build.go
@@ -24,7 +24,7 @@
 	ctx.JoinPath = path.Join
 	ctx.IsAbsPath = path.IsAbs
 	ctx.SplitPathList = func(list string) []string { return strings.Split(list, ":") }
-	ctx.IsDir = func(path string) bool { return false }
+	ctx.IsDir = func(path string) bool { return path == "." }
 	ctx.HasSubdir = func(root, dir string) (rel string, ok bool) { return "", false }
 	ctx.ReadDir = dir.readDir
 	ctx.OpenFile = dir.openFile