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

vlib/sqlite/README.md

@@ -1,39 +1,4 @@
 ## Description
 
-`sqlite` is a thin wrapper for [the SQLite library](https://sqlite.org/), which in turn is
-"a C-language library that implements a small, fast, self-contained,
-high-reliability, full-featured, SQL database engine."
-
-# Install SQLite Dependency
-
-Before you can use this module, you must first have the SQLite development
-library installed on your system.
-
-**Fedora 31**:
-
-`sudo dnf -y install sqlite-devel`
-
-
-**Ubuntu 20.04**:
-
-`sudo apt install -y libsqlite3-dev`
-
-
-**Windows**:
-- Download the source zip from [SQLite Downloads](https://sqlite.org/download.html)
-- Create a new `sqlite` subfolder inside `v/thirdparty`
-- Extract the zip into that folder
-
-# Performance Tips
-
-When performing a large amount of database calls (i.e. INSERTS), significant
-performance increase can be obtained by controlling the synchronization and journal modes.
-
-For instance:
-```v
-import sqlite
-
-db := sqlite.connect('foo.db') or { panic(err) }
-db.synchronization_mode(sqlite.SyncMode.off)
-db.journal_mode(sqlite.JournalMode.memory)
-```
+The `sqlite` module has been moved to `db.sqlite`.
+Update your code to do: `import db.sqlite` instead.