[ffigen] Fixed wrapper not found error when running pub run ffigen (#42)

Possibly fixes #41 (Needs to be tested after publishing).

- Using `dart:cli` to resolve package uri and get wrapper.c for running `pub run ffigen:setup`.
- If `dart:cli` is unavailable, fallback tries to work up the current directory to find `.dart_tool/package_config.json` and get rootUri of `package:ffigen` and find wrapper.c from there.