[BUG] Entrypoint resolver is not compatible with node folder mappings

[iMagdy]

Bug Report Quick Checklist

• [x] I am on the latest version of Snowpack & all plugins.
• [x] I use package manager npm
• [x] I run Snowpack on OS Mac
• [x] I run Snowpack on Node.js v12+

Describe the bug

Using any node module that makes use of node's subpath folder mappings (regardless whether the deprecated syntax or the new one) will throw an error:

Package "xxxx" exists but package.json "exports" does not include entry for "./folder_path/file.js"

Example for map (node's deprecated):

"./directives/": {
    "default": "./directives/"
},

Example for map (new):

"./directives/*.js": {
    "default": "./directives/*.js"
},

To solve this issue, this node module must use absolute file path in the imports instead of folder mappings.

To Reproduce

1. Install an node module like lit (https://lit.dev) which have exports map, then try to import any directive, for example dp import { cssMap } from 'lit/directives/css-map.js'
2. See the error.

   Package "lit" exists but package.json "exports" does not include entry for "./directives/until.js"

Expected behavior

Should parse folder mappings correctly with both deprecated and new syntax.

[cmditch]

Also hitting this issue with using @snowpack/web-test-runner-plugin and Google's firebase on import 'firebase/compat/firestore'

[cmditch]

@iMagdy have you found a workaround for this by chance? This has been a blocker for me to get web-test-runner working.

Testing in snowpack has proven to be very difficult, haven't had luck w/ jest either.