Parse subpath exports from exports sugar

[huozhi]

Today we need to add a package.json for a subpath folder if you want to build based on that working directory, for instance:

// pkg/a/package.json

{
   "name": "pkg-a",
   "exports": {}
}

In this way, you can build the assets based on the subpath pkg/a and generate output according to "exports" path in that package.json. Then we refer those outputs in the toplevel package.json for the corresponding subpath imports.
But this might confuse some tooling to understand the structure of the library better. Since node.js introduced the exports sugar, any subpath needs to provide a package.json just for getting assets output paths. bunchee should be able to auto detect it

Proposal

Introducing a new exports paths auto-detection feature for top level package.json.

Example

Assume we have a toplevel package foo with package.json which contains a subpath import foo/die

{
   "name": "foo",
   "exports": {
      "./die": {
          "import": "...",
          "require": "...",
          "types": "...",
      }
   }
}

Then you have your source folder of foo/die/ with entry file foo/die/index.js. In this case bunchee should read the output paths infomation from exports field if the current build working directory matches the any exports path