New, more consistent package directory structure.

- Packages now live in <package_name>/package.py

- spack.packages refactored to use a PackageDB object instead of
  monolithic module.

- Implementation of mock_packages_test.py is greatly simplified

- Added test to exercise install/uninstall code because that wasn't
  covered by existing tests and kept breaking.
Modify create and edit to work with new package format.
Add a test for package names
Move packages and mock packages to /var/spack
Update docs for new package structure.