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

examples/database/mysql.v

@@ -1,4 +1,4 @@
-import mysql
+import db.mysql
 
 fn main() {
 	mut conn := mysql.Connection{

examples/database/orm.v

@@ -1,6 +1,6 @@
-import sqlite
-import mysql
-import pg
+import db.sqlite
+import db.mysql
+import db.pg
 
 [table: 'modules']
 struct Module {

examples/database/psql/customer.v

@@ -1,6 +1,6 @@
 module main
 
-import pg
+import db.pg
 
 const dash = '----------------------------------------------------------------'
 

examples/database/sqlite.v

@@ -1,4 +1,4 @@
-import sqlite
+import db.sqlite
 
 fn main() {
 	db := sqlite.connect(':memory:')!