Fix: try harder to resolve .mjs files for the browser entries

[ndelangen]

Closes #21125

What I did

@tmeasday found that the issue was caused by duplicate bundling of addon preview code.

This was because sometimes the .mjs file was resolved, and sometimes the .js file was resolved.
I've added code to try to find .mjs files for resolved files we know will end up in the browser.

I've tested this in the reproduction, and this fixed the issue.

I suspect this will also have quite a positive effect on the bundle-size, and potentially even on build-time.
Good stuff.

[tmeasday]

Will this not be a problem if the addon does not have a .mjs file in dist? We can't assume that all addons will do that, right?

Are you sure that following through more with the technique of stripping the extension (which previously we were doing for the managerEntries but not previewAnnotations) isn't the right solution here?

[ndelangen]

@tmeasday the better solution would be to maybe use enhanced-resolve or something.

I did verify this worked. I was planning on introducing better support for export maps at some point via some method (again enhanced-resolve comes to mind). In fact I experimented with it for a bit, but couldn't make it work within the time-box, and so abandoned it in favor of this solution.

[tmeasday]

OK, makes sense then.