Fix SPM integration

[diegotori]

Looks like the header files weren't referenced properly in the iOS platform channel. Also upgraded Pigeon to the latest version.

Addresses issue #94.

[diegotori]

LGTM. Skipping dart format failures, since this is not critical for fixing this issue.

[Gustl22]

I kind of wonder, why this problem happened: I added the source files via podspec:

wakelock_plus/wakelock_plus/ios/wakelock_plus.podspec

Line 17 in f54b6d9

s.public_header_files = 'wakelock_plus/Sources/wakelock_plus/include/**/*.h'

We should not need relative imports.

The same way it's done in flutter (e.g.:):
https://github.com/flutter/packages/blob/main/packages/video_player/video_player_avfoundation/darwin/video_player_avfoundation.podspec

[Gustl22]

This might be the same problem as:
https://github.com/firebase/flutterfire/pull/11472/files, but I also wonder, why the official packages don't have this problem, probably because they use the same messages.g.m class for ios and macos (just a guess)