Get a 'graphical' tree-overview of installed packages

Generates a ascii-art-graphical tree of packages and their dependancies.
For example for itk-3.2_1:

      [~] edwin@k7>pkg_tree tk-8
      tk-8.3.3
        |\__ tcl-8.3.3_3 (unknown)
        |\__ imake-4.1.0 (unknown)
        |\__ freetype2-2.0.4
         \__ XFree86-libraries-4.1.0 (unknown)

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