This changes variant display to use a much more legible format, and to use screen space
much better (particularly on narrow terminals). It also adds color the variant display
to match other parts of `spack info`.
Descriptions and variant value lists that were frequently squished into a tiny column
before now have closer to the full terminal width.
This change also preserves any whitespace formatting present in `package.py`, so package
maintainers can make easer-to-read descriptions of variant values if they want. For
example, `gasnet` has had a nice description of the `conduits` variant for a while, but
it was wrapped and made illegible by `spack info`. That is now fixed and the original
newlines are kept.
Conditional variants are grouped by their when clauses by default, but if you do not
like the grouping, you can display all the variants in order with `--variants-by-name`.
I'm not sure when people will prefer this, but it makes it easier to tell that a
particular variant is/isn't there. I do think grouping by `when` is the better default.
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