The requirements/edx/private.txt file is for dev's own private package
needs. There are two installation mechanisms in edx-platform, and
neither handled the file properly:
- `paver install_prereqs` had the wrong file name. The file was moved
almost three years ago, and paver wasn't kept up.
- `make requirements` used `private.*` which included private.in, which
pip-sync balks at.