Hack for Windows users to compensate for Graphviz "feature".

Recent versions of Graphviz no longer set the PATH variable on Windows.
Hence Scyther can fail to find dot.exe even though Graphviz was appropriately
installed.

This patch is a hack to try and locate dot.exe in the more common locations.
We currently have a hardcoded search through:

  C:\Program Files\Graphviz*
  C:\Program Files (x86)\Graphviz*

This is clearly fragile. Obviously, nobody should be solving Graphviz' problem
in such an ugly way. Change drives or languages and it stops working.

Until Graphviz provides at least an alternative environment variable (GVPATH?)
this hack will help the bulk of our users to get things up and running smoothly.
Cleanup of obsolete module import.