Improve update sites management

[pcdavid]

There are several issues regarding how the p2 repos are published, exposed and (not) cleaned up.
See these bugzillas for details:

• Please provide "latest" composites for update sites or at least clean them up.
• EMFt downloads needs a clean up (opened against EMF Transaction, but relevant for Validation too).

[pcdavid]

See https://eclipse.dev/justj/?page=tools for p2 repo publication and management in replacement of ad hoc and brittle shell scripts.