Update iOS flutterEngine libraryURI

[johypark]

Description of what this PR is changing or adding, and why: I have to import another file in main and use package prefix in libraryURI reference Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• [x] This PR is marked as draft with an explanation if not meant to land until a future stable release.
• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [ ] This PR uses semantic line breaks of 80 characters or fewer.