Simple patch to fix building debian package

[Alan Bateman]

Mandy Chung wrote:
> jpkg fails to create the debian package for the JDK modules
> because it uses '_' which is no longer a valid character
> in the debian version.
>
> This patch simply fixes to return the jigsaw version and
> also checks for valid characters and throws exception if
> the jigsaw version was changed to use any invalid character.
> In addition, only include the mandatory dependencies in
> the Depend statements for the debian package.
>
> Webrev:
>    http://cr.openjdk.java.net/~mchung/jigsaw/webrevs/jigsaw-deb-pkgs/
>
> Mandy
>
Thanks for fixing this, I've been building lately with 
BUILD_PACKAGES=false because of this. The changes look okay to me. I 
think computeProvides can also be changed to deps.substring(2).

-Alan