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