Log Message:
Bump pkgdepgraph to 2.1.  New stuff:

* label the graph and the subgraphs (feature)
* rework subgraph identification in the main loop (feature)
* better handling of recoloring graphs that contain subgraphs (bugfix)
* be a little more picky about what *might* be a pkg directory (bugfix
  via seb)
* fix a bug in error reporting in canonification routine (bugfix via
  seb)
* add comments (bugfix?  feature?)