[video_player] Define clang module for iOS

[jmagman]

Description

• Limit the supported podspec platform to iOS so tests don't run (and fail) for macOS.
• Define the module by setting DEFINES_MODULE in the podspec. See CocoaPod modular headers docs.
• Explicitly set VALID_ARCHS to x86_64 for the simulator. See No way to add passing test_spec that doesn't cause build warning "Mapping architecture armv7 to i386. Ensure that this target's Architectures and Valid Architectures build settings are configured correctly for the iOS Simulator platform" CocoaPods/CocoaPods#9210.

Related Issues

See flutter/flutter#41007.

Tests

• Remove video_player from the list of packages to skip when linting the podspec. This will at minimum make sure video_player can be imported into a project without compilation errors.

Checklist

• 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 (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[cyanglaz]

LGTM