Consolidate DAG traversal in traverse.py, support DFS/BFS (#33406)

This PR introduces breadth-first traversal, and moves depth-first traversal
logic out of Spec's member functions, into `traverse.py`.

It introduces a high-level API with three main methods:

```python
spack.traverse.traverse_edges(specs, kwargs...)
spack.traverse.traverse_nodes(specs, kwags...)
spack.traverse.traverse_tree(specs, kwargs...)
```

with the usual `root`, `order`, `cover`, `direction`, `deptype`, `depth`, `key`,
`visited` kwargs for the first two.

What's new is that `order="breadth"` is added for breadth-first traversal.

The lower level API is not exported, but is certainly useful for advanced use
cases. The lower level API includes visitor classes for direction reversal and
edge pruning, which can be used to create more advanced traversal methods,
especially useful when the `deptype` is not constant but depends on the node
or depth. 

---

There's a couple nice use-cases for breadth-first traversal:

- Sometimes roots have to be handled differently (e.g. follow build edges of
  roots but not of deps). BFS ensures that root nodes are always discovered at
  depth 0, instead of at any depth > 1 as a dep of another root.
- When printing a tree, it would be nice to reduce indent levels so it fits in the 
  terminal, and ensure that e.g. `zlib` is not printed at indent level 10 as a 
  dependency of a build dep of a build dep -- rather if it's a direct dep of my
  package, I wanna see it at depth 1. This basically requires one breadth-first
  traversal to construct a tree, which can then be printed with depth-first traversal.
- In environments in general, it's sometimes inconvenient to have a double
  loop: first over the roots then over each root's deps, and maintain your own
  `visited` set outside. With BFS, you can simply init the queue with the
  environment root specs and it Just Works. [Example here](https://github.com/spack/spack/blob/3ec73046995d9504d6e135f564f1370cfe31ba34/lib/spack/spack/environment/environment.py#L1815-L1816)

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