pip-compile includes irrelevant constraints from pip contraints file

[derek-miller]

pip-compile treats constraints as requirements.

https://pip.pypa.io/en/stable/user_guide/#constraints-files

Steps to replicate

1. Create a constraints.txt file with the following contents:

click==6  # Irrelevant package
itsdangerous==0.24

2. Create a requirements.in file with the following contents:

-c constraints.txt
Flask

2. run pip-compile

Expected result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements.txt requirements.in
#

Flask==0.11.1
itsdangerous==0.24
Jinja2==2.8               # via flask
MarkupSafe==0.23          # via jinja2
Werkzeug==0.11.11         # via flask

Actual result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements.txt requirements.in
#

click==6.0
Flask==0.11.1
itsdangerous==0.24
Jinja2==2.8               # via flask
MarkupSafe==0.23          # via jinja2
Werkzeug==0.11.11         # via flask

[derek-miller]

@nvie I took a stab at this, I would appreciate it if you could take a look.

[derek-miller]

@nvie Any chance you could take a look at this?

[derek-miller]

@nvie?

[derek-miller]

Github's PR user interaction is quite terrible and marked this as Merged when I was trying to rebase, forcing me to open another PR. See #471.