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 changes the representation of maps in the interpolator from the
dotted flatmap form of a string variable named "var.variablename.key"
per map element to use native HIL maps instead.
This involves porting some of the interpolation functions in order to
keep the tests green, and adding support for map outputs.
There is one backwards incompatibility: as a result of an implementation
detail of maps, one could access an indexed map variable using the
syntax "${var.variablename.key}".
This is no longer possible - instead HIL native syntax -
"${var.variablename["key"]}" must be used. This was previously
documented, (though not heavily used) so it must be noted as a backward
compatibility issue for Terraform 0.7.
The nodes it adds were immediately skipped by flattening and therefore
never had any effect. That makes the transformer effectively dead code
and removable. This was the only usage of FlattenSkip so we can remove
that as well.
