hostinfo: move packageType out to platform-specific files

Change-Id: I3236b3d4e2376dd7e2482c2562817b1b6f44872e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo_darwin.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2022 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.
+
+//go:build darwin
+// +build darwin
+
+package hostinfo
+
+import (
+	"os"
+	"path/filepath"
+)
+
+func init() {
+	packageType = packageTypeDarwin
+}
+
+func packageTypeDarwin() string {
+	// Using tailscaled or IPNExtension?
+	exe, _ := os.Executable()
+	return filepath.Base(exe)
+}