fix pip index handling

[hrbonz]

Fix handling of the proper long index option (--index-url) for pip install.

As far as I can tell, pip install uses -i and --index-url (see documentation), the pip search accepts --index long option. Those options are added by pip-tools when using pip-compile.

I'm wondering if the packages should not be associated with that index, following https://docs.pipenv.org/advanced.html#specifying-package-indexes

Change mentioned in #856

Example requirements.txt coming out of pip-compile:

--index-url https://pypi.example.com/xxx/+simple/
Django==1.9.5
myprivatepkg==1.1.5

[nateprewitt]

Thanks!