add link to install marketplace next to matomo plugins when marketplace is not currently installed

[diosmosis]

In case users manually install Matomo plugins without the Marketplace installed, they may be confused when there's no automatic update for the plugin available.

To avoid support requests around this, we can:

• when the marketplace is not installed, on the WP plugins admin page, detect any installed Matomo plugins, and add a link to the plugin description to the marketplace setup wizard
• detect if there are new versions available for these plugins, and display that info as well to motivate users to install the marketplace plugin