Since [37221] the upload form is added to every plugin install screen via `install_plugins_upload()`. Previously the form was added through the `install_plugins_upload` (alias of `install_plugins_$tab`) action which allowed plugin authors to replace the form. This restores the previous behaviour.
* Add the form only to non-upload plugin install screens.
* Replace `install_plugins_upload()` with the `install_plugins_upload` and `install_plugins_pre_upload` actions.
* Remove `$upload_tab_class` and add a CSS class for the current tab to `.wrap`
* Adjust CSS selectors and toggle the whole container to support upload without an `upload-plugin` class.
Props DavidAnderson, ocean90.
Fixes#37495.
Built from https://develop.svn.wordpress.org/trunk@38172
git-svn-id: http://core.svn.wordpress.org/trunk@38113 1a063a9b-81f0-0310-95a4-ce76da25c4cd