Revert "Hide State created from MetaState and its provides"

One problem is that useful informations can be contained in
metastate. For instance, InstallOnDebian state contains the list of
package names and this list is exported in the XML tree.

This reverts commit 9a9cf07bb8a2aa5d3f0a2f247338f59977fb161d.
Remove all provides from states that are implementations of a
MetaState.
Add documentation about MetaState