Revisit pubspecs and general information on pub.dev

[kahest]

Platforms

Currently, we don't specify the supported platforms (only exception is sentry-flutter using the plugin declarations). Pub.dev has auto-detection but that can be flaky. Consider adding the platforms explicitely in pubspec.

Licenses

The main package currently uses BSD while the other packages use MIT. Is this on purpose/correct?

More tbd

Verify pub.dev info, especially stuff that's auto-detected, find out if we can/should add explicit info in pubspec.

[ueman]

The main package is BSD since it was originally developed by Google as far as I remember.