fix(core, ios): update Package.swift and header imports for backwards compatibility

[russellwheatley]

Description

• Cannot find a way to get firebase-ios-sdk version from remote repo, in the future, we can make a script for updating sdk version in preCommit hook of melos version.
• Updated headers to make firebase core source file compatible with cocoapods and Swift.

Related Issues

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Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.