directory structure change to work with glide

glide has its own requirements. My previous workaround caused me some
code checkin errors. Need to fix this.

weed/filer/flat_namespace/flat_namespace_store.go

@@ -0,0 +1,9 @@
+package flat_namespace
+
+import ()
+
+type FlatNamespaceStore interface {
+	Put(fullFileName string, fid string) (err error)
+	Get(fullFileName string) (fid string, err error)
+	Delete(fullFileName string) (fid string, err error)
+}