Package dag implements a directed acyclic graph task runner with deterministic teardown. it is similar to package errgroup, in that it runs multiple tasks in parallel and returns the first error it encounters. Users define a Runner as a set vertices (functions) and edges between them. During Run, the directed acyclec graph will be validated and each vertex will run in parallel as soon as it's dependencies have been resolved. The Runner will only return after all running goroutines have stopped.