Inconsistent refreshManifestCache and missed manifest file for modules, component and plugins

Fix the issue #355 about inconsistent refreshManifestCache.

It was looking for a fixed file name for manifest, instead of use the installation behavior, to use the first valid manifest file found. 

If someone install an extension with a manifest.xml instead of mod_myext.xml, it's ok, until the user refresh the manifest cache or try to add a module (joomla will not find the right manifest file).

 For modules, components and plugins.