Extract manifest target metadata This parses target metadata from manifest files into an in-memory representation, verifying its validity in the process. Change-Id: I71a060126f92d20e641f7800c76da8361ad0b847 Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/125130 Reviewed-by: Erik Gilling <konkers@google.com> Commit-Queue: Alexei Frolov <frolv@google.com>