net/{interfaces,netmon}, all: merge net/interfaces package into net/netmon

In prep for most of the package funcs in net/interfaces to become
methods in a long-lived netmon.Monitor that can cache things.  (Many
of the funcs are very heavy to call regularly, whereas the long-lived
netmon.Monitor can subscribe to things from the OS and remember
answers to questions it's asked regularly later)

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: Ie4e8dedb70136af2d611b990b865a822cd1797e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -33,7 +33,6 @@ import (
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/connstats"
-	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netcheck"
 	"tailscale.com/net/neterror"
 	"tailscale.com/net/netmon"
@@ -936,7 +935,7 @@ func (c *Conn) determineEndpoints(ctx context.Context) ([]tailcfg.Endpoint, erro
 	eps = c.endpointTracker.update(time.Now(), eps)
 
 	if localAddr := c.pconn4.LocalAddr(); localAddr.IP.IsUnspecified() {
-		ips, loopback, err := interfaces.LocalAddresses()
+		ips, loopback, err := netmon.LocalAddresses()
 		if err != nil {
 			return nil, err
 		}

wgengine/router/ifconfig_windows.go

@@ -20,7 +20,7 @@ import (
 	"golang.org/x/sys/windows"
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"tailscale.com/health"
-	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tstun"
 	"tailscale.com/util/multierr"
@@ -110,7 +110,7 @@ func monitorDefaultRoutes(tun *tun.NativeTun) (*winipcfg.RouteChangeCallback, er
 }
 
 func getDefaultRouteMTU() (uint32, error) {
-	mtus, err := interfaces.NonTailscaleMTUs()
+	mtus, err := netmon.NonTailscaleMTUs()
 	if err != nil {
 		return 0, err
 	}