terraform graph: By default, produce a compact graph with only resources in it

[apparentlymart]

This fixes #14511.

I actually wrote the first two commits here as part of a new round of work on #30937, but after implementing Graph.ResourceGraph I remembered that a graph just of the relationships between resources -- without all of the other intermediaries -- was the main idea I heard about when researching #14511 many years ago.

Given that the terraform graph command is an ancillary debugging/learning tool rather than part of the main Terraform workflow, I find it pretty unlikely that we'd prioritize any work directly on it in the near future, but since my work here already got most of the way there I figured I might as well make a small effort to close that old issue at the same time, while I'm working in this area anyway.

The effect of this PR, then, is to change the default behavior of terraform graph to produce a reduced graph that includes only the resources (both resource and data) described in the configuration, grouped by which module they belong to. Any intermediate nodes such as input variables and output values are not directly included, but indirect dependencies through them are preserved and shown as if they were direct dependencies between resources.

The idea of prioritizing resources here is to roughly align with how we prioritize information in other parts of the CLI, such as in terraform plan and terraform show which both spend most of their output showing data about resources and de-emphasize (or ignore entirely) most other kinds of objects in Terraform.

This also incorporates another very old terraform graph improvement idea, which is to arrange the graph in reverse order of the dependency arrows so that a sighted viewer can read the image from left to right to see approximately what order the operations might happen in, with the first operations on the far left and the last operations on the far right. This aligns with the left-to-right reading order of most languages using the latin alphabet, which is the alphabet Terraform uses for its own identifiers/etc. Anyone who would prefer to have it oriented the other way (with the first operations on the far right) can change the rankdir argument to be "LR" instead after they've generated a graph.

This will change behavior for anyone who is routinely running terraform graph with no other options. That command was intentionally excluded from the Terraform v1.x compatibility promises in anticipation of making changes like this one, and the old default behavior is still available here as terraform graph -type=plan. If we move forward with this, I'll add an upgrade note to the changelog describing how to obtain the previous default behavior.

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While in the area anyway I also did a small amount of modernization of the terraform graph command and its tests, now using the "streams" abstraction to write to stdout instead of using the legacy "Ui" object.

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I mentioned above that the first two commits here are really for #30937. The connection there is that resources are the main object type that performs actions that can potentially need to be deferred due to unknown values, and so having a summarized graph with just the relationships between resources makes it more efficient to ask the question "does this resource depend on any other resources that have already been deferred?". But more on that will follow in later PRs, once I've made more progress on the details there.

Because this was a "while I was in the area" sort of thing, I don't expect to have time to respond to non-trivial feedback, but of course if anyone has any big concerns about this then we can always put it back on the shelf and try again another time!

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