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Instead of having softwares install yarn, unify this in nodejs stack.

Yarn usage is similar to nodejs usage, if a specific version is needed, software should use macro to expose which yarn version to use, example:

[yarn]
<= yarn-1.17.3

Then sections can use yarn by having ${yarn:location}/bin/ in their path.

yarn will use the default [nodejs], so to another nodejs version, the same pattern can be used:

 [nodejs]
 <= nodejs-10.6.0