Move |require "tsort"| to the file where |TSort| is used
When operating on multiple YaST modules, sort them topologically first

The sorting is done by |ybc_deps|. This allows e.g. |yk ybc all| to work
on a clean machine, because YaST modules will be processed in the order
of their dependencies.
Merge pull request #245 from yast/tsort-yast-modules

[Review] Request from '' @ 'yast/ycp-killer/tsort-yast-modules'