Add virtual information on DAG edges

[alalazo]

see #34821

fixes #34886

Currently virtual dependencies are used only during concretization to select a provider. No annotation is kept on edges to reflect that a leaf node is providing a virtual dependency. This PR adds virtual information on edges, so we can keep track of which spec is providing what virtual both in memory and in the JSON / YAML representation.

The way edge attributes are added is generic, so to reuse the same representation later when the use_variant will be implemented, see spack/seps#2. A sample representation is:

        "dependencies": [
          {
            "name": "cmake",
            "hash": "iqpoju67adln3yqzvqzccrtkjmpc666m",
            "parameters": {
              "deptypes": [
                "build"
              ],
              "virtuals": []
            }
          },
          {
            "name": "openmpi",
            "hash": "p23v372ij7bomrarjrhcv4uw6ieahjt3",
            "parameters": {
              "deptypes": [
                "build",
                "link"
              ],
              "virtuals": [
                "mpi"
              ]
            }
          },
          {
            "name": "pkgconf",
            "hash": "i4avrindvhcamhurzbfdaggbj2zgsrrh",
            "parameters": {
              "deptypes": [
                "run"
              ],
              "virtuals": [
                "pkgconfig"
              ]
            }
          },
          {
            "name": "zlib",
            "hash": "nizxi5u5bbrzhzwfy2qb7hatlhuswlrz",
            "parameters": {
              "deptypes": [
                "build",
                "link"
              ],
              "virtuals": []
            }
          }
        ],

Modifications:

• Add a parameters attribute to DependencySpec, which can hold arbitrary attributes
• Modify Spec.to_node_dict to output information on virtuals in the `"dependencies" section
• Add unit tests (round-trip specs / check annotation)
• Modified DependencySpec to hold an arbitrary number of attributes (98b9806)
• Reconstruct virtuals from old specfile formats (0b2db30)

[haampie]

Is it really necessary to have a dict here? Personally I would keep redundancy & the number of indirections small so that we don't hit even more Python slowness. (One of the bottlenecks is creating the edges when deserializing the db, so if a normal prop works that's likely better than a string and hashmap lookup -- ok, to be fair, we do go from json string -> dict -> props, so in a sense it could also be faster)

[alalazo]

Using properties is a step towards adding custom attributes on edges. This is needed for "use variants" as proposed in spack/seps#2 and can be implemented on top of #35322, so 2 PRs from here.

[haampie]

I think you want to restrict to direct deps here?

[alalazo]

That should be done already, since the loop below selects only outgoing edges from the root.

[haampie]

Sure, but in case vspec is not a direct dep you walk the whole dag? And what if it is a dependency of a dependency?

[alalazo]

Turned out the reconstruction was incomplete. See 3f1205a