Refactor to use import map

[rwe]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

The module exported an entrypoint do-not-use-conditional-color as a way of using the module system to determine a runtime default value. However, a subpath import is more appropriate for this, because they're private and don't introduce a public entrypoint:
https://nodejs.org/api/packages.html#subpath-imports

[wooorm]

That was what I wanted to use, but I was worried about their support.
I’ve now released import maps in a few packages and it seems to work