The dot format generation was done with a mix of code from the terraform
package and the dot package. Unify the dot generation code, and it into
the dag package.
Use an intermediate structure to allow a dag.Graph to marshal itself
directly. This structure will be ablt to marshal directly to JSON, or be
translated to dot format. This was we can record more information about
the graph in the debug logs, and provide a way to translate those logged
structures to dot, which is convenient for viewing the graphs.
This adds the proper logic for "disabling" providers to the new apply
graph: interolating and storing the config for inheritance but not
actually initializing and configuring the provider.
This is important since parent modules will often contain incomplete
provider configurations for the purpose of inheritance that would error
if they were actually attempted to be configured (since they're
incomplete). If the provider is not used, it should be "disabled".
