Move parent POM to a separate module

I wanted to separate module aggregation from model inheritance.  In
future the topmost XMvn module is going to include configuration that
is not supposed to be inherited by submodules.
Generate binary tarball for main artifact
Skip installation if package is not installable

One good reason to skip package installation is unavailability of
suitable installer for its packaging type.  The whole reactor should
build successfully and installation of the artifact with unsupported
packaging type skipped if it is marked as not installable.