consider new build options for making p2 sites

[jonahgraham]

At the moment LSP4J has a bunch of custom build logic to handle building the various Maven/p2 outputs - there is a new feature that is being added to Tycho that should make this easier. Jetty is trying it out now and LSP4J should consider moving to this too jetty/jetty.project#6403 (comment)

[laeubi]

@jonahgraham you can try it out with the latest tycho snapshot already, any feedback is appreciated.