The contents of the cupertino_icons generation repo should be moved to flutter/packages

Per the comments here.

The generated dart file should be distributed as part of the cupertino_icons package

Currently the generated dart file is part of the cupertino library instead of the cupertino_icons package. With the font and the codepoint map living in different packages, there doesn't seem to be a way to properly version the dart codepoint binding, or to test it. Example: I would like to fix an incorrectly mapped codepoint in cupertino_icons < 1.08 but since the dart file lives in cupertino it would break code that's using older versions (https://github.com/flutter/flutter/pull/146806/files#diff-6622a57feb8a0b6d1674c2cad83821872010e3c8005be91079c3d34d466a0f91)

This could be a breaking change but not doing it is kinda bad too because we can't fix incorrect codepoints without breaking apps that are using older versions of cupertino_icons.