requirements file parsing with normal option defaults

[qwcode]

1. parse with defaults set as they are (vs forcing to None) and adjust the logic to match; the result is simpler.
2. Due to No instructions for how to install pip #1, we can remove some hairy "format_control" hacks that arose in response.
3. Due to No instructions for how to install pip #1, let's relax the parsing and allow:
   • multiple options per line
   • any supported option on a line with a requirement (not just --install-option/--global-option, although they are the only options that are scoped to the requirement)
     pip install --editable and pip install clash for namespace packages #3 may be debatable, but it seems ok to me.

cc @rbtcollins , since you had to work around this for format_control parsing

[rbtcollins]

Looks much cleaner to me.

[qwcode]

@dstufft @pfmoore, can I get your thoughts on #3 from the description, since it's somewhat major.

we could check when option values are != to the defaults, but it's only a partial solution. we'd still fail to detect when:

• an option happens to set the default value
• when there's multiple of the same option

We could possibly do some low-level overrides in optparse to detect this, but it seems like the wrong path.

The important thing is that requirements can still only be one per line, and --install-option/--global-option on those lines are scoped to the requirement.

[pfmoore]

#3 seems OK to me. It mostly allows things that I don't think people will (or should) ever do.

I'd assume that the extra behaviour would be undocumented. So we have the option to treat weird behaviour as "don't do that" rather than as bugs to fix. I'd prefer that over trying to document and support all the corner cases.