fix: better handle concurrent builds

[dsanders11]

Checklist

• npm install && npm run lint && npm test passes
• tests are included
• commit message follows commit guidelines

Description of change

Fixes #3165, which unfortunately was not actually fixed by #3170. I'm not sure #3170 had any effect on the issue, as my original changes in #2846 which were extended to all platforms in #3710 were targeting a specific issue on Windows, and did not aim to fix (or know of) the issue in #3165.

The second commit on this branch adds a test which repros the issue being reported in #3165, namely that header files can be seen as partially written when multiple builds are happening concurrently. A failing test run before the other commits on this branch can be seen here.

The remainder of the commits refactor copyDirectory to fix the issue, fix another concurrent build issue I came across during testing, and adds more test coverage. This refactor also allows for dropping exponential-backoff as a dependency.