VersionConflict on setup_requires requirements
A long-standing annoyance with setuptools and distribute, is that if a package has some requirement in setup_requires, but the user already has an old version of that distribution in their site-packages, then the installation will fail with a VersionConflict.
This behavior is antithetical to the use-case for setup_requires, where all build-time requirements are made available to setup.py, installing them into the current directory as eggs, if necessary, but otherwise isolated from disturbing anything in site-packages or affecting run-time behavior.
If a package required by setup_requires is more recent than the installed version, we should still be able to download the more resent version, install it locally as an egg, and activate it on the path in preference to the incompatible installed version.
This mostly just requires a slight addition to WorkingSet.resolve() specifically for this use case.