xrevshlib: add option to consider only given package/subpackage

[triallax]

Sometimes the SO version of a library from only the main package or of one subpackage is bumped, so having xrevshlib return the shlib-dependent packages for that package only would be quite useful.

Edit: thinking about this more, it would also be nice if xrevshlib could take shared library names as arguments, instead of a package name.