Add `Package::exports()` to replace `Package::namespace().exports`

[DavisVaughan]

From #1306 (comment)

This should be a Package::exports() query mirroring File::exports(). I thought I had implemented it already, > but can't find it (maybe it was abandoned at some point).

There will be some complex resolution in the future (e.g. for regex exportPattern), so we should call a single > method everywhere exports are needed.

It'd return a vector of names that you can then resolve with top-level Package::resolve().

And maybe namespace() should be private?

I had some outstanding questions about this from this request:

• What do we do with other existing calls to namespace().exports? We have a few, like

  ark/crates/ark/src/lsp/diagnostics.rs

  Line 201 in e62a675

  for export in &pkg.namespace().exports {

• Do the documented symbols get added to Package::exports()'s output?

  ark/crates/ark/src/lsp/diagnostics.rs

  Lines 965 to 973 in 59ba153

  	// Start from explicit `NAMESPACE` exports
  	let mut exports = package.namespace(context.db).exports.clone().into_vec();
  	// Add all documented symbols. This should cover documented datasets to avoid some
  	// false positives in favor of allowing some false negatives. This is admittedly a
  	// bit of a stopgap!
  	if let Some(index) = package.index(context.db) {
  	exports.extend(index.names().iter().cloned());
  	}

• What is the exact return value Vec<Name>?

• Is Package::resolve() actually relevant here?