Only show one search form for the plugin installer.

The field dropdown now appears in the filter bar only when doing a search.

see #28785.

Built from https://develop.svn.wordpress.org/trunk@29597


git-svn-id: http://core.svn.wordpress.org/trunk@29371 1a063a9b-81f0-0310-95a4-ce76da25c4cd