A good example of this is when the plugin being updated is currently installed as 'Plugin-Name' but the canonical directory is 'plugin-name', but it can also occur when the plugin is installed in 'super-cool-plugin' and it's canonical name is 'average-plugin'.
Merges [32570] to the 4.2 branch.
Fixes#32465
Built from https://develop.svn.wordpress.org/branches/4.2@33301
git-svn-id: http://core.svn.wordpress.org/branches/4.2@33273 1a063a9b-81f0-0310-95a4-ce76da25c4cd
8e96288144