Plugin Directory: Make compatible with Jetpack 10.5

[kraftbj]

Jetpack has updated the Search module to move it into a package.

The refactoring, at least, breaks the requires in class-plugin-search.php:
https://meta.trac.wordpress.org/browser/sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/class-plugin-search.php#L71

Need to update those to the new locations and confirm that the migration of the Helper class does not cause any additional issues.

I can handle the testing and patch, but I lack commit to meta.