The following are all valid values for the `"exports"` map (https://nodejs.org/api/packages.html#packages_exports_sugar):

```
"exports": {
  ".": {
    "import": "./index.js"
  }
}
```

```
"exports": {
  "import": "./index.js"
}
```

```
"exports": "./index.js"
```

Currently ipjs only supports the first version, this PR adds support for the other two as well.