Versions for java-virtual-machine

[Julio]

Can a virtual package have a version (to be set by a 'real' package that 
implements it)? If so, it'd be useful to have java-virtual-machine packages to 
set their jdk-compliance versions (1.0, 1.1, 1.2) when installing (or being set 
by update-alternatives), since it's better to bind some packages on virtual 
machines of specific versions.