Broke out PlayWatchService and AssetsClassLoader to be shared

These files can be used in multiple contexts.  In particular, they will
be needed by the forth-coming forked-run support needed by Activator
based on sbt-server.

[Updated] Added play.sbtplugin.run.PlayWatchService back with deprecation warnings

* Also, improved stand-alone sbt scripted test setup
Actually, even if they are build against the same Scala version you still get two different artifacts (There is an explicit name given to disambiguate the artifacts).  This trick is based on the one we used in atmos and echo.