Move "logger" package to under types, now that we have it.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/logger/logger.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package logger defines a type for writing to logs. It's just a
+// convenience type so that we don't have to pass verbose func(...)
+// types around.
+package logger
+
+type Logf func(fmt string, args ...interface{})