build --save missing 'import' field in '.' export entry

[brainkim]

Bug

Running libuild build --save generates an exports map in the root package.json where the "." entry is missing the "import" field, while other entries have it:

"exports": {
    ".": {
      "types": "./dist/src/crank-editable.d.ts"
    },
    "./crank-editable": {
      "types": "./dist/src/crank-editable.d.ts",
      "import": "./dist/src/crank-editable.js"
    }
}

The "." entry should also have "import": "./dist/src/crank-editable.js".

Reproduction

cd packages/crankeditable
npx libuild build --save
cat package.json  # "." export missing "import"

This causes bundlers like esbuild to fail resolving the package when symlinked locally.

Expected

".": {
  "types": "./dist/src/crank-editable.d.ts",
  "import": "./dist/src/crank-editable.js"
}