improve support for package.json exports when building up import maps

[thescientist13]

Type of Change

Enhancement

Summary

As part of integrating with various ecosystem libraries, it has been observed that additional cases need to be accounted for when handling the exports object from a package.json

Details

Observed and reported use cases that we should validate against

• walk package error if @spectrum-web-components/action-menu is installed even if not used #1309
• pnpm shoelace hackathon thescientist13/greenwood-lit-ssr#18
• feature/issue 32 migrate to greenwood AnalogStudiosRI/api#44

While we're here, we should probably also try and improve our error handling here, like making sure only strings get added to the import map, and good use of try / catch