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

From https://github.com/posit-dev/ark/pull/1306#discussion_r3512781622

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 https://github.com/posit-dev/ark/blob/e62a6753c72569f70480ffd5d3f1b5f3258ccd50/crates/ark/src/lsp/diagnostics.rs#L201

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

  https://github.com/posit-dev/ark/blob/59ba153406c227b4e0cac2515bbf02c7eb1d741b/crates/ark/src/lsp/diagnostics.rs#L965-L973

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

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

Start by comparing the requested Package::exports() query with File::exports(), then inspect the existing namespace().exports calls in crates/ark/src/lsp/diagnostics.rs. Resolve whether documented symbols belong in the Vec result and whether Package::resolve() is needed; update all callers so exports are obtained through the new method.