fixes failure to uninstall editables in certain edge cases with symlinks

If TMPDIR is on a different filesystem, this has the potential to lose
file ownership info. In general that shouldn't be an issue; seems to
be the best we can do.

Factor out the requirement handling into pip.req, and most of the remaining utility functions into pip.util

Move all things pyflakes complains about into backwardcompat; extract the rmtree onerror logic into a separate function

Extracted the Command class (and helpers); fix -E now that pip is a package; extracted some virtualenv stuff; extracted exceptions; tweak tests

Fix the invocation of pip, now that virtualenv includes pip and that installed version has to be overrided