Fixup bootstrap script to point to https://pypi.python.org and fixup
plumbing to allow said-same for distribution resolution.

Also fixup an over-aggressive assert.  Some distributions, like
distribute 0.7.3, just serve as a dependency alias and export no top
level packages.
Merge pull request #156 from twitter/jsirois/pants/egg_woes

Fixup bootstrap script to point to https://pypi.python.org and fixup handling of meta-distributions