Update Swift Package manifest to fix x86_64-swift-linux-musl build

[ilia3546]

Summary

Updated the Swift Package manifest to use platform-conditional dependencies instead of compile-time #if canImport(Compression) checks, fixing build issues on Linux platforms including x86_64-swift-linux-musl.

Problem

The previous approach using #if canImport(Compression) had a critical issue:

1. Compile-time evaluation limitation: canImport(Compression) is evaluated when Package.swift is compiled, not when the actual target is built. This created unreliable
   behavior on Linux, especially with musl libc.

2. Implicit platform handling: The approach relied on compile-time checks rather than explicit platform declarations, preventing SPM from correctly handling
   platform-specific dependencies.

Reproduction

To reproduce the issue, try building any SPM executable package with ZIPFoundation as a dependency:

swift build --swift-sdk x86_64-swift-linux-musl

The build would fail with the old manifest structure.

Solution

The new approach uses SPM's native .when(platforms:) conditional dependency system:

1. Explicit platform targeting: Clearly defines which platforms need CZLib ([.linux, .openbsd]) vs. native Compression framework.

2. Proper dependency resolution: SPM resolves dependencies correctly before compilation, knowing exactly what's needed for each platform.

3. Single unified target structure: One target definition with conditional dependencies instead of duplicated configurations.

Changes

• Removed #if canImport(Compression) conditional compilation
• Unified target definitions into a single structure
• Added CZLib as a conditional dependency for Linux and OpenBSD only
• Applied _GNU_SOURCE conditionally for Linux platforms
• Maintained backward compatibility across all platforms