Plugin search: Wrap results in a form to fix pagination's `paged` input field.

props jesin, ocean90.
see #18724, for trunk.
Built from https://develop.svn.wordpress.org/trunk@29829


git-svn-id: http://core.svn.wordpress.org/trunk@29594 1a063a9b-81f0-0310-95a4-ce76da25c4cd