Add command to execute single target

This implements a basic "do" subcommand which runs a single target in
the build graph with all of its dependencies.

Change-Id: I75bd6558662fe0eafa5e1a82a855f112b63aa524
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126910
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
6 files changed
tree: 6addd8fdb75b23085e3e90bf72fe89129a91e18a
  1. examples/
  2. qg/
  3. qg-cli/
  4. tools/
  5. .gitignore
  6. Cargo.toml
  7. rustfmt.toml