Since the DAG package was lifted from Terraform, its contents are more
than what we need for now, so this commit cleans-up the package to keep
only the currently needed parts of code.
If we need to support more in the future, we can revert this commit, or
pickup the changes again from Terraform.
The implementation of the DAG as extracted from Terraform relied on a
Root vertex being injected into the graph as the last node to visit.
This is used as a sanity check for Terraform, but doesn't apply to our
use-case for now, as we are always executing everything and have no need
for this root node.
Instead, we change how Validate operates so it does not error in case
there is no valid root node for the graph, but enables us calling it to
check for self-referencing edges, and circular dependencies.
The dag package is a port over from Terraform to Packer, changing what
little there was to fit our current dependency ecosystem.
Most of the changes are on the type of diagnostics returned, as
Terraform has its own type for them, while we rely on hcl's Diagnostics.
Other than that, the functionality is essentially equivalent, and the
code was barely touched.
