pip install --egg break's pip's dependency resolution

[qwcode]

pip install --egg remove's pip's use of --single-version-externally-managed, which is the basis for pip's dependency resolution, which underlies it's "requirements file" feature.

e.g. do this: pip install --egg peppercorn==0.2 deform

you should end up with peppercorn v0.2, but instead you get v0.4

[qwcode]

original PR #541 (and issue that motivated it #3)
related issue #1037