Rules N813 and N814 prevented import statements like this:

  xml.etree.ElementTree as et
  xml.etree.ElementTree as ET

But both of those seem pretty reasonable.  We see no reason to require any camelcase import to be imported "as" a camelcase word.

W503 and W504 are issued for line breaks before or after binary
operators (respectively). This allows either approach (explicitly
instructing flake8 to ignore both of these cases).

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.