[bootstrap] Build TS refs in parallel

[tylersmalley]

Results in a small (~7%) reduction in the build time. However, it does mean that projects referenced outside of x-pack might be doing twice the work at the expense of build time.

I first began by combining both tsconfig.refs.json to see if there would be an improvement in the build time. However, that didn't make a measurable difference.

[spalger]

I'm a little worried about race conditions between the projects in the default config and their similar references from xpack, if the build for src/core is running in both process at the same time for instance do we know that TS will handle it correctly? Do we know that TS doesn't mess with the output files from one process or the other?

[tylersmalley]

@spalger, I am not sure. @restrry, do you know?

[spalger]

I'm seeing a larger and exciting 45 second improvement when running this branch, tested with

find src x-pack packages -name '*.tsbuildinfo' | xargs rm && yarn kbn run kbn:bootstrap -i kibana

With four runs on both branches I got average times of 276s on master and 231s on this branch, which is an awesome little speed-up.

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 2869e12

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[mshustov]

yes, I removed parallel builds in #79438
I'd suggest merging both tsconfig.refs.json in a single file instead.

[tylersmalley]

Closing in favor of #90537