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

vlib/orm/README.md

@@ -112,7 +112,7 @@ result := sql db {
 
 ### Example
 ```v ignore
-import pg
+import db.pg
 
 struct Member {
 	id         string [default: 'gen_random_uuid()'; primary; sql_type: 'uuid']

vlib/orm/orm_sql_or_blocks_test.v

@@ -1,5 +1,5 @@
 import os
-import sqlite
+import db.sqlite
 
 struct User {
 	id   i64    [primary; sql: serial]

vlib/orm/orm_test.v

@@ -1,9 +1,9 @@
 // import os
 // import term
-// import mysql
-// import pg
+// import db.mysql
+// import db.pg
 import time
-import sqlite
+import db.sqlite
 
 struct Module {
 	id           int       [primary; sql: serial]