group variants more efficiently

[buddly27]

When working with a high number of variants, it would help speeding up the resolution if we added a step to combine equivalent ones. Let's consider the following example:

• def1

  {
  "identifier": "def1",
  "variants": [
      {
          "identifier": "V2",
          "requirements": ["foo >= 2, < 3"]
      },
      {
          "identifier": "V1",
          "requirements": ["foo >= 1, < 2"]
      }
  ]
  }

• def2

  {
  "identifier": "def2",
  "variants": [
      {
          "identifier": "V2",
          "requirements": ["foo >= 2, < 3"]
      },
      {
          "identifier": "V1",
          "requirements": ["foo >= 1, < 2"]
      }
  ]
  }

Combination order will depends on position of the nodes in the graph, but in the worst case scenario, this order would be a complete waste of time:

• [def1[V1], def2[V2]]
• [def1[V2], def2[V1]]
• [def1[V2], def2[V2]]

Instead, it would be great to have some logic to smartly combine [def1[V2], def2[V2]] and [def1[V1], def2[V1]] as they both share the same requirements