Option --no-use-pep517 not supported in requirement files

Environment

• pip version: 19.1
• Python version: 3.6.7, 2.7.15
• OS: any

Description Specifying the option --no-use-pep517 in a requirements file results in

ERROR: Invalid requirement: --no-use-pep517 docopt pip: error: no such option: --no-use-pep517

Expected behavior Package is installed not using PEP 517

How to Reproduce

1. Have this requirements file:

--no-use-pep517 docopt

1. Run pip install -r requirements.txt (removing the option form the file and passing it directly works normal)

Output

pip install -r requirements.txt 
Usage: pip [options]

ERROR: Invalid requirement: --no-use-pep517 docopt
pip: error: no such option: --no-use-pep517

This Issue regards specifying --no-use-pep517 inside a single requirement file. It is related to #6433, where it fails when we have nested requirement files and specify the option outside of them, i.e. give it directly as an argument to the pip invocation.

[sbidoul]

I removed the awaiting PR label, following the two comments above.

[brainwane]

@pradyunsg Not to derail this particular issue, but I think this is not a good first issue; it might be a good second or third issue, but a contributor who is new to Python packaging is going to have trouble understanding specifiers, PEP 517, and so on. Removing the tag.