Deduplicate package Swift settings

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Summary

• remove the initial per-target swiftSettings assignment from Package.swift
• rely on the existing package target loop to apply ExistentialAny, MemberImportVisibility, and InternalImportsByDefault once to every target

Validation

• ran git diff --check
• verified Package.swift has a single ExistentialAny setting application path

Note: I could not run SwiftPM tests in this environment because Swift/Xcode is not installed on the Windows host.