Replace tuples with better suited data structures for server references

This PR introduces the `ServerReferenceExport` struct to replace
`(Ident, Atom, Atom)` tuples for tracking exported identifiers. It also
replaces the `Vec<(Atom, Atom)>` `export_actions` with an `FxIndexMap`
for `reference_ids_by_export_name`, enabling more efficient lookups in
upstack PRs. There are no functional changes.

Author: unstubbable