warn if removing `svelte` resolve override would break a package

[benmccann]

Describe the problem

Right now the svelte field is resolved first. We should eventually remove this and defer to Vite which will resolve exports first

Describe the proposed solution

Add dependency on @alloc/resolve.exports to resolve the exports. If exports exist and resolve to different place than svelte field then warn.

Alternatives considered

Discussed on Discord the the maintainers group and I think this is what we arrived at. I won't bother repeating the discussion

Importance

nice to have

[dominikg]

love the idea of testing if resolve returns a different result with or without the svelte field.

We would have to issue a warning too for the case if removing the svelte field would fail resolving entirely if there is no exports map or other entries. Right now this package.json is legal for svelte libraries

{
  "name": "some-lib",
  "svelte": "index.js",
  "type": "module"
}

There are also hybrid libraries that export node code as well as svelte files and some might leverage different "main" and "svelte" indexes for this.

not sure we need the external lib for this, can't we just leverage vite's this.resolve or something ?

[benmccann]

@dummdidumm suggested continuing to support the svelte field, which seems reasonable to me. I think the thing we really need to change is that it has precedence over exports. In that case we wouldn't need any warnings for the example file above.

I didn't realize Vite exported this.resolve. Using that instead of an external library sounds good to me.