Correcting some errors for core package creation

This saves all fields in the package xml, and lets create a core-compliant language pack.

It is not a final approach to the general packages creation where one should have the choice between a 3pd extension lang or langs pack and core.
Also, for core packs, translator should have the choice to include css,  js (in media) for example

Even with this patch we still have some issues for core packs as not all ini files are included (plg, tpl, and also some files for no apparent reason: mod_version, mod_multilangstatus.