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>