## What

I propose adding a `file_exists()` check before calling
`is_plugin_inactive()` in the `admin_footer` action inside `db.copy`.

## Why

If the SQLite plugin is inactive for some reason but the `db.php`
drop-in file exists, the logic automatically activates the plugin. This
works only when the SQLite plugin is installed into the `plugins/`
directory and throws warnings if plugin is installed into the
`mu-plugins/` directory.

The new guard ensures that the plugin is installed in `plugins/` before
running activation logic.

## Testing

1. Install WordPress
2. Install the SQLite plugin from the .org directory
3. Activate the plugin - confirm `db.php` drop-in is created
4. Deactivate the plugin by bypassing normal logic:
```
wp option delete active_plugins
```
6. Confirm `db.php` still exists
7. Reload WP Admin
8. Confirm the plugin is auto-activated

## Summary
Define `WP_MYSQL_ON_SQLITE_PATH` constant in
`wp-pdo-mysql-on-sqlite.php` so that dependent projects can reliably
determine the path to the SQLite driver loader file.

…tension" (#335)

Reverts #329

Migrate the database file from a possible legacy path on connection,
including associated WAL, SHM, and journal files.