Extract supported requirement file options automatically

[chrahunt]

What's the problem this feature will solve?

Currently the documentation for what options are possible in requirements files are hand-maintained in the docs, but actually enforced in the code. Maintaining the list in both places is error-prone.

Describe the solution you'd like

Automatically extract the supported options from the lists of options in pip._internal.req.req_file here.

The approach should be similar to what we do for command option help here. We should also make sure to clearly separate the general options, which must be on their own line in a requirements file, from the requirement-specific options, which must be on the same line as a requirement.

Alternative Solutions

• Continue to manually maintain the lists in both places

Additional context

• From #7386 (comment)