Allow pip freeze to emit VCS strings

[Naddiseo]

• Pip version: 8.1.1
• Python version: Python 3.5.1
• Operating System: Ubuntu 16.04

Description:

I'm trying to build a bunch of wheels of each of the requirements for my project and put them in a docker container so that I don't have to install all the build requirements on said container. I thought I would be able to do this with the following:

pip freeze > build_requirements.txt
pip wheel -w ./wheels -r build_requirements.txt

Usually, I think this would work, but I also have two requirements that come from git repositories that we've forked and changed slightly to fit our needs. So, for example, we have something like:

$ cat requirements.txt
module1
git+https://github/user/repo@mybranch#egg=package
$ pip freeze > build_requirements.txt && cat build_requirements.txt
module1==1.0.0
package==1.0.0-custom

However, we don't publish our custom changes to PyPI, so building the wheel wont work since the package doesn't exist. If I make the git repo editable via "-e", wheel wont build it.

So, the two ways of fixing this would be

1. Allow wheel to build editable entries
2. Or, allow freeze to output CVS strings

(This might be related to or duplicate of #3379)

[xavfernandez]

1. was solved via allow creating wheels for editable packages #3695
2. is a duplicate of pip freeze does not list repo url #609