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/util/net.go

@@ -0,0 +1,21 @@
+package util
+
+import (
+	"fmt"
+	"net"
+)
+
+// CanBindPort - checks if we can bind to this port or not
+func CanBindPort(port int) (bool, error) {
+	addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf("127.0.0.1:%d", port))
+	if err != nil {
+		return false, err
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		return false, err
+	}
+	_ = l.Close()
+	return true, nil
+}