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>

doctor/ethtool/ethtool_linux.go

@@ -8,7 +8,7 @@ import (
 	"sort"
 
 	"github.com/safchain/ethtool"
-	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/set"
 )
@@ -21,7 +21,7 @@ func ethtoolImpl(logf logger.Logf) error {
 	}
 	defer et.Close()
 
-	interfaces.ForeachInterface(func(iface interfaces.Interface, _ []netip.Prefix) {
+	netmon.ForeachInterface(func(iface netmon.Interface, _ []netip.Prefix) {
 		ilogf := logger.WithPrefix(logf, iface.Name+": ")
 		features, err := et.Features(iface.Name)
 		if err == nil {