modules: prepare for v2 release

- update go.mod \w v2
 - update all import paths

Signed-off-by: Alexander Bezzubov <bzz@apache.org>

internal/code-generator/generator/samplesfreq.go

@@ -12,7 +12,7 @@ import (
 	"strconv"
 	"text/template"
 
-	"github.com/src-d/enry/internal/tokenizer"
+	"github.com/src-d/enry/v2/internal/tokenizer"
 )
 
 type samplesFrequencies struct {