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
