pip freeze -r loses url information

[Ivoz]

When a package is specified by a url in a requirements file, pip freeze -r will lose that url. i.e:

$ cat req.txt 
https://www.djangoproject.com/download/1.6.2/tarball/#egg=Django
$ pip freeze -r req.txt 
Django==1.6.2
## The following requirements were added by pip --freeze:
wsgiref==0.1.2

i.e pip freeze -r req.txt > req.txt would lose information on the round trip. If the package was not in the package index, it would no longer be installable from the new file in a new environment.

[qwcode]

as for parsing the pkg url's see #1689

[xavfernandez]

Duplicate of #452