Export map without '.' entry throws on self-referenntial deep imports

[MylesBorins]

package.json

{
  "name": " bada",
  "exports": {
    "./boom": "./lib/powder.mjs"
  }
}

index.mjs

import 'bada/boom'

Will throw with the error

Error: Cannot find package '$PACAKGE_NAME' imported from ...

Adding a dot export to the package.json makes thing work as expected

package.json
```json
{
  "name": " bada",
  "exports": {
   ".": "./ohnoes.lol",
    "./boom": "./lib/powder.mjs"
  }
}

Seems like this might be to spec (as currently written). I'm not sure that we want to enforce a "." main if folks don't need one (thinking about top level application), but at the very least we likely want to throw with a better error... lost some time debugging this.

[guybedford]

Seems like this may be related to #30602.

[guybedford]

Fixed in #31009.