Read Flutter version from pubspec.yaml

[brainwo]

Add option to read Flutter version from pubspec.yaml when it's specified.

Fallback to .puro.json for channel name or specific fork version.

[pingbird]

The version constraint in pubspec.yaml is usually quite loose, I could add a warning that tells the user what the latest version that satisfies the constraints is

[aramix]

if flutter version is not specified in pubspec.yaml, you can always find it in pubspec.lock