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/check_other_dhcp_windows.go

@@ -0,0 +1,11 @@
+package dhcpd
+
+import "fmt"
+
+func CheckIfOtherDHCPServersPresentV4(ifaceName string) (bool, error) {
+	return false, fmt.Errorf("not supported")
+}
+
+func CheckIfOtherDHCPServersPresentV6(ifaceName string) (bool, error) {
+	return false, fmt.Errorf("not supported")
+}