sourcedr: Serialize dependency graph with pickle

This commit adds `pickle` sub-command to ninja.py.  This command
serializes parsed dependency graph into a pickle object file, which is
much easier to load.

Test: ninja.py pickle out/combined-sailfish.ninja -o sailfish.pickle \
  && list_system_file_from_vendor_source.py sailfish.pickle

Change-Id: I4e4e4f828bf485c75daafb78ddb35d93b1f37abd