Python code refactorings.

This commit refactors the python code in diskimage_builder slightly.  This is
in preparation for a larger review that adds more functionality to the python
code, namely the ability to apply elements to the current system as opposed to
a chroot.  Further, the refactorings can stand on their own for better clarity.
They include:
 - renaming elements.py to element_dependencies.py.  Adds clarity about the
   purpose of this module.
 - updating other code for this rename.
 - move tests into a tests submodule.

Change-Id: I5519cc52398e442b24e33802bae42070d64b0c1d
Merge "Python code refactorings."