Related posts contain non-visible WooCommerce Products

#### Steps to reproduce the issue

Products can be visible, hidden or search (meaning they must only show up in searches), this is stored in the _visibility meta key with a corresponding value 'visible', 'hidden', or 'search'.
Jetpack's related posts doesn't appear to take this into account, and thus happily shows hidden products.

To test this, create a hidden product and see it show up in Related posts from another product details page.

#### What I expected

Jetpack shouldn't do this. Considering Jetpack and WooCommerce come from the same Automattic stable, we can expect them to be aware of each other. Products that are not marked as visible should not be shown.
Logic: if there is a _visibility meta key, remove the post unless it has value of 'visible'.

#### What happened instead

Hidden products are visible in Related Posts.

Start by tracing Jetpack Related Posts and how it handles WooCommerce products, then reproduce the issue with a hidden product as described. Done means products with a _visibility value other than "visible" no longer appear in Related Posts, while visible products still do.

Written by the indexing model from the issue text.