Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/dhcpd/os_windows.go

@@ -0,0 +1,13 @@
+package dhcpd
+
+import (
+	"errors"
+	"net"
+
+	"golang.org/x/net/ipv4"
+)
+
+// Create a socket for receiving broadcast packets
+func newBroadcastPacketConn(bindAddr net.IP, port int, ifname string) (*ipv4.PacketConn, error) {
+	return nil, errors.New("newBroadcastPacketConn(): not supported on Windows")
+}