Add utility to collect headers from google/mediapipe

[craiglabenz]

Summary

Adds a script to crawl google/mediapipe for the headers necessary to run ffigen for the flutter_mediapipe packages. This command assumes the two repositories (this one, google/flutter-mediapipe, and the main one, google/mediapipe) are both checked out on the machine running the command. google/mediapipe is assumed to be adjacent to google/flutter-mediapipe, though a CLI option exists to override this assumption.

Usage

$ cd packages/build_cmd
$ dart bin/main.dart headers [-s /path/to/google/mediapipe]

TODOs:

• [ ] Add tests
• [ ] Possibly rename folder. Can we do better than build_cmd? (build feels appropriate, but for its connotations and default inclusion in .gitignore files)

[craiglabenz]

This PR is also ready for final review, @Piinks 🎉