provides can list a `*` node for all platforms

[coveralls]

Pull Request Test Coverage Report for Build 7570597873

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 83.384%

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Totals
Change from base Build 7422033929:	0.3%
Covered Lines:	2218
Relevant Lines:	2588

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💛 - Coveralls

[mxcl]

I elected not to release this and reverted it instead.

Rationale: this is not an additive change and I don’t want people to be in a situation where pkgx was working and then stops working without them updating pkgx.

[jhheider]

more likely it should would like env:

provides:
  - bin/normal-bin
  - darwin:
      - bin/bin-macos
  - linux:
      - bin/bin-linux

mildly ugly, but my linter thinks it's valid yaml.

[mxcl]

true this would work, nice idea