fix(arborist): clean up orphaned scoped store entries in linked strategy

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor.

Under --install-strategy=linked, each external package is extracted into a content-addressed store entry keyed by name@version-hash.
For scoped packages the key contains the unescaped scope slash, so the entry nests one extra directory level: node_modules/.store/@scope/pkg@version-hash/node_modules/@scope/pkg.
The store key itself spans two path segments.

When a scoped dependency is updated or removed, the orphan-cleanup pass in #cleanOrphanedStoreEntries failed to remove the stale store entry, so the store accumulated dead scoped entries on every update.
This never happened for unscoped packages.

The root cause is that the cleanup made a single-segment assumption in two places.
When collecting valid keys from the ideal tree, loc.split('/')[2] returned just @scope instead of the full @scope/pkg@version-hash key.
When enumerating on-disk entries, the non-recursive readdir(storeDir) returned the @scope directory as a single entry and never descended into the per-version subdirectories inside it.
The orphan filter then compared @scope on disk against @scope in the valid set, so the scope directory was always considered valid and never swept, no matter how many stale versions it contained.

Fixed by reconstructing the full two-segment key for scoped entries when collecting valid keys, and by descending one level into each @scope directory when enumerating on-disk entries so the real per-version entries are listed and compared as full keys.
Both sides are compared in forward-slash form, matching how ideal-tree locations are already normalized, so resolve reconstructs the on-disk path correctly on POSIX and Windows.

Removing the last scoped orphan under a scope would leave an empty @scope directory behind, so after removing orphans the pass now prunes any scope directory that has become empty.

References

Fixes #9440