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/helpers_test.go

@@ -0,0 +1,14 @@
+package util
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSplitNext(t *testing.T) {
+	s := " a,b , c "
+	assert.True(t, SplitNext(&s, ',') == "a")
+	assert.True(t, SplitNext(&s, ',') == "b")
+	assert.True(t, SplitNext(&s, ',') == "c" && len(s) == 0)
+}