There are now separate flake8 configs for core vs. packages:
- core has a smaller set of flake8 exceptions
- packages allow `from spack import *` and module globals
- Allows core to take advantage of static checking for undefined names
- Allows packages to keep using Spack tricks like `from spack import *`
  and dependencies setting globals for dependents.

* Properly ignore flake8 F811 redefinition errors
* Add unit tests for flake8 command
* Allow spack flake8 to work on systems with older git
* Skip flake8 unit tests for Python 2.6 and 3.3

- Exempt overlong URL lines from checks.
- Omit some of the more painful PEP items.