Data Liberation plugin v7

URL updater plugin – fix plugin updates (#4)

Before this PR, the `url-updater` plugin supported installing the plugin
but it often failed to install the new version. For example, if the
zipped directory name changed, or if the downloaded plugin did not
declare a different version number. With this PR, we're switching to a
custom plugin updater based on `WordPress\Filesystem` components. This
means we can easily support more data sources, e.g. cloning the plugin
directly from a git repository.

## Testing instructions

1. Go to http://127.0.0.1:9400/wp-admin/admin.php?page=remote-installer
2. Install a zipped plugin from any URL
3. Click "Change URL" and paste in a different plugin URL
4. Confirm the original plugin was removed and the new plugin is now
installed in its place