fix(ios): preserve packager source map before compose overwrites it

[sbycrosz]

1. On iOS combinedSourceMapOutput is the same path as bundleSourceMapOutput, so compose-source-maps.js overwrites the Metro packager map in place
2. Android avoids this because it composes to a separate .map.
3. This change copies the packager map to .packager.map before compose runs, matching Android, so tooling that needs the chunk/debug ID (e.g. Uploading source-maps to Sentry / posthog-cli) can still recover it✌️