Integrate option value derivation into help.

[benjyw]

• The normal help now shows value derivation for each displayed option.
• Replaces the old options goal, which was overly complex.
• Gets rid of OptionTracker. The parser itself now does the tracking, by
  adding a details field to RankedValue.
• Streamlines Parser._compute_value (although it is still huge).
• Fixes a bug where the default value we displayed was sometimes the
  current value, not the HARDCODED default.
• Also removes the defaulting to os.environ/sys.argv in various places
  in the options system. You now have to be explicit about passing those through.
• Removes the affordance for the args maybe not including the binary name
  in tests. Now args[0] must always be the binary name (that is, ./pants).

A future change will provide a way to dump all help info, including the
value derivation history, out to JSON.

[ci skip-rust-tests]

[benjyw]

[stuhood]

Great stuff! Thanks.

The normal help now shows value derivation for each displayed option.

Should it maybe only render the value if it is different from the default?

[benjyw]

Should it maybe only render the value if it is different from the default?

I had it that way at first, but I found it subjectively worse than being consistent.

[Eric-Arellano]

Wonderful! Excellent work.

[Eric-Arellano]

Maybe save this for a followup so that it shows up in the changelog as a distinct change. Stu has been planning to remove recursive options.