vlib: move the mysql/sqlite/pg/mssql modules under vlib/db (#16820)

tutorials/building_a_simple_web_blog_with_vweb/README.md

@@ -182,7 +182,7 @@ Add a SQLite handle to `App`:
 
 ```v oksyntax
 // blog.v
-import sqlite
+import db.sqlite
 import vweb
 
 struct App {

tutorials/building_a_simple_web_blog_with_vweb/code/blog/blog.v

@@ -2,7 +2,7 @@ module main
 
 import vweb
 import time
-import sqlite
+import db.sqlite
 import json
 
 struct App {