QPluginLoader: fix loading of plugins with a relative file name

QPluginLoader: fix loading of plugins with a relative file name

This makes QT_PLUGIN_PATH / QCoreApplication::libraryPaths() actually work,
as a search path for plugins, when apps look for a specific plugin by name.

To make it possible to write portable code (unlike the current QPluginLoader
unittest), let QPluginLoader figure out the extension, too.

Change-Id: I895d597d7cb05ded268734bc5f313f32d8d12cb9
Reviewed-by: Lars Knoll <lars.knoll@digia.com>