control, ipn, tailcfg: remove golang.org/x/oauth2 dep, add tailcfg.Oauth2Token

golang.org/x/oauth2 pulls in App Engine and grpc module dependencies,
screwing up builds that depend on this module.

Some background on the problem:
https://go.googlesource.com/proposal/+/master/design/36460-lazy-module-loading.md

Fixes tailscale/corp#1471

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

ipn/handle.go

@@ -8,8 +8,8 @@ import (
 	"sync"
 	"time"
 
-	"golang.org/x/oauth2"
 	"inet.af/netaddr"
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 )
@@ -155,7 +155,7 @@ func (h *Handle) StartLoginInteractive() {
 	h.b.StartLoginInteractive()
 }
 
-func (h *Handle) Login(token *oauth2.Token) {
+func (h *Handle) Login(token *tailcfg.Oauth2Token) {
 	h.b.Login(token)
 }