Rename some type
* method -> methodType
* type -> typeType
* version -> versionType
* client -> clientType
* language -> languagesType
* files -> filesType

Correct handling of
* updateservers with child
* updateservers wihtout child element

Adding support for validation of packges and library using files at https://github.com/joomla/joomla-cms/tree/master/tests/_data/installer_packages

Adding groupType to module.xsd for supporting a predefined set of group or any string (The installer will create new folder names for group names that do not exist yet.)