This change adds support for setting `PULUMI_PREFER_YARN` to true to opt-in to preferring `yarn` over `npm` when installing Node.js dependencies (and publishing Policy Packs). If `PULUMI_PREFER_YARN` is truthy, but `yarn` cannot be found on `$PATH`, we fallback to using `npm`. If `npm` can't be found on `$PATH`, we provide a more helpful error message.
If we don't process and report the stderr of `npm install`, the output
is "orphaned" during error condition, and only something like "exit code
1" is reported.
This commit will implement the core business logic of `pulumi policy
publish` -- code to boot an analyzer, ask it for metadata about the
policies it contains, pack the code, and transmit all of this to the
Pulumi service.
