Using export map instead of custom property in `package.json`

[webcomponents]

Using export map instead of custom property in `package.json`

[thepassle]

For the foreseeable future, not all packages will make use of export maps yet, so I think we'll have to have two ways for now:

If a package uses export maps:

 "exports": {
   "customElements": "custom-elements.json",
 }

If a package does not use export maps yet, add a field in the package.json:

"customElements": "custom-elements.json"

[justinfagnani]

As discussed in our meeting about this, a manifest could be included in package exports if the package intends it to be an importable JSON module from a consuming package, but this isn't a primary use case.

The use case we most need to cover is for a source analyzing tool to be able to discover a package.json and custom elements manifest given a base path and a specifier, resolving the specifier within the source's available node_modules tree, not the tool's. This may require implementing part of Node's module resolution algorithm as a utility function. We'll need this whether or not the custom elements manifest is exported.

I think we can close this as an issue that's part of the manifest spec and figure it out in tools.