new-installer: Make findExtensions() return a key-value array

new-installer: Make findExtensions() return a key-value array

Now we return e.g.:

    array( 'Cite', 'LiquidThreads' );

Instead return:

    array(
        'Cite' => null,
        'LiquidThreads' => $some_config
    );

Where $some_config is a yet-to-be determined configuration. It's
intended for e.g. sourcing SQL tables that the extension needs.