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/dnsfilter/README.md

@@ -0,0 +1,70 @@
+# AdGuard Home's DNS filtering go library
+
+Example use:
+```bash
+[ -z "$GOPATH" ] && export GOPATH=$HOME/go
+go get -d github.com/AdguardTeam/AdGuardHome/dnsfilter
+```
+
+Create file filter.go
+```filter.go
+package main
+
+import (
+    "github.com/AdguardTeam/AdGuardHome/dnsfilter"
+    "log"
+)
+
+func main() {
+    filter := dnsfilter.New()
+    filter.AddRule("||dou*ck.net^")
+    host := "www.doubleclick.net"
+    res, err := filter.CheckHost(host)
+    if err != nil {
+        // temporary failure
+        log.Fatalf("Failed to check host '%s': %s", host, err)
+    }
+    if res.IsFiltered {
+        log.Printf("Host %s is filtered, reason - '%s', matched rule: '%s'", host, res.Reason, res.Rule)
+    } else {
+        log.Printf("Host %s is not filtered, reason - '%s'", host, res.Reason)
+    }
+}
+```
+
+And then run it:
+```bash
+go run filter.go
+```
+
+You will get:
+```
+2000/01/01 00:00:00 Host www.doubleclick.net is filtered, reason - 'FilteredBlackList', matched rule: '||dou*ck.net^'
+```
+
+You can also enable checking against AdGuard's SafeBrowsing:
+
+```go
+package main
+
+import (
+    "github.com/AdguardTeam/AdGuardHome/dnsfilter"
+    "log"
+)
+
+func main() {
+    filter := dnsfilter.New()
+    filter.EnableSafeBrowsing()
+    host := "wmconvirus.narod.ru" // hostname for testing safebrowsing
+    res, err := filter.CheckHost(host)
+    if err != nil {
+        // temporary failure
+        log.Fatalf("Failed to check host '%s': %s", host, err)
+    }
+    if res.IsFiltered {
+        log.Printf("Host %s is filtered, reason - '%s', matched rule: '%s'", host, res.Reason, res.Rule)
+    } else {
+        log.Printf("Host %s is not filtered, reason - '%s'", host, res.Reason)
+    }
+}
+```