vfs: factor the vfs cache into its own package

vfs/vfscommon/path.go

@@ -0,0 +1,12 @@
+package vfscommon
+
+import "path/filepath"
+
+// FindParent returns the parent directory of name, or "" for the root
+func FindParent(name string) string {
+	parent := filepath.Dir(name)
+	if parent == "." || parent == "/" {
+		parent = ""
+	}
+	return parent
+}