Plugins endpoint: add icon information

Currently we fetch icon information separately from `api.wordpress.org`. This means that after getting the list of endpoints, for each plugin we have to make one extra request to get the plugin info, and another one to get the actual icon.

It'd be nice if the icon URL was included in the plugins response.

Locate the plugins endpoint response and the code that separately requests plugin information and icons from api.wordpress.org. Confirm how the response is assembled and identify the relevant tests, if present. Done means the plugins response includes each icon URL and the separate icon requests are no longer required.

Written by the indexing model from the issue text.