heroku-buildpack-python: Install with `-e` in `requirements.txt` results in missing dependency

Hi!

Up front I’d like to say thanks for all hard work on the build pack. I’d also like say that my experience managing dependencies with python is minimal so I apologize if I’ve done something to cause the issue myself.

We have two submodules in our project repository that we install using the following convention in our requirements.txt (reproduced from our repo):

# this is the default included as a part of the python build pack from heroku
bob-builder==0.0.5

# install the softix submodules
-e lib/softix/core
-e lib/softix/cli

Previously this worked just fine but now when attempting to make use of the CLI tool from our app we’re getting the following stack trace:

$ heroku run bash                         
Running bash on  wastastaging... up, run.4739 (Free)
~ $ softix --help
Traceback (most recent call last):
  File "/app/.heroku/python/bin/softix", line 6, in <module>
    from pkg_resources import load_entry_point
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3017, in <module>
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3003, in _call_aside
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3030, in _initialize_master_working_set                                                                                                 File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 659, in _build_master
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 967, in require
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 853, in resolve
pkg_resources.DistributionNotFound: The 'softixcli' distribution was not found and is required by the application

To aid in reproduction of the issue I’ve created a repository that can be deployed to a new instance:

https://github.com/johnbender/heroku-buildpack-python-issue

I noticed that I had to run the deploy once or twice and also attempt to run the command once or twice to get the above error. 😦

Our current workaround is to reference an older version of the buildpack:

$ heroku buildpacks                                                                                                                                                 
=== Buildpack URLs
1. https://github.com/heroku/heroku-buildpack python#1bccc03b30d31c314452f2648ebb0296f1e5f155
2. heroku/nodejs