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/assets/content.go.tmpl

@@ -3,7 +3,7 @@ package data
 import (
 	"regexp"
 
-	"github.com/src-d/enry/data/rule"
+	"github.com/src-d/enry/v2/data/rule"
 )
 
 var ContentHeuristics = map[string]*Heuristics{

internal/code-generator/assets/header.go.tmpl

@@ -1,3 +1,3 @@
-// Code generated by github.com/src-d/enry/internal/code-generator DO NOT EDIT.
+// Code generated by github.com/src-d/enry/v2/internal/code-generator DO NOT EDIT.
 // Extracted from github/linguist commit: {{ getCommit }}