fix: support Node.js type stripping by importing from .ts files

[achingbrain]

To (for example) run tests without prior typescript compilation, use the .ts extension for relative imports.

This is just to speed local development in a future aegir release, the built output of the project does not change.

Fixes #2812

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[tabcat]

thanks 🚀