go.mod: upgrade wireguard-windows, de-fork wireguard-go

Pull in the latest version of wireguard-windows.

Switch to upstream wireguard-go.
This requires reverting all of our import paths.

Unfortunately, this has to happen at the same time.
The wireguard-go change is very low risk,
as that commit matches our fork almost exactly.
(The only changes are import paths, CI files, and a go.mod entry.)
So if there are issues as a result of this commit,
the first place to look is wireguard-windows changes.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/magicsock/legacy.go

@@ -16,11 +16,11 @@ import (
 	"sync"
 	"time"
 
-	"github.com/tailscale/wireguard-go/conn"
-	"github.com/tailscale/wireguard-go/tai64n"
 	"golang.org/x/crypto/blake2s"
 	"golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/crypto/poly1305"
+	"golang.zx2c4.com/wireguard/conn"
+	"golang.zx2c4.com/wireguard/tai64n"
 	"inet.af/netaddr"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/types/key"

wgengine/magicsock/magicsock.go

@@ -27,9 +27,9 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/tailscale/wireguard-go/conn"
 	"golang.org/x/crypto/nacl/box"
 	"golang.org/x/time/rate"
+	"golang.zx2c4.com/wireguard/conn"
 	"inet.af/netaddr"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/derp"

wgengine/magicsock/magicsock_test.go

@@ -26,9 +26,9 @@ import (
 	"time"
 	"unsafe"
 
-	"github.com/tailscale/wireguard-go/device"
-	"github.com/tailscale/wireguard-go/tun/tuntest"
 	"golang.org/x/crypto/nacl/box"
+	"golang.zx2c4.com/wireguard/device"
+	"golang.zx2c4.com/wireguard/tun/tuntest"
 	"inet.af/netaddr"
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"