bake: fix group resolution

[crazy-max]

follow-up #881

If a target has the same name as a group, it's resolution is skipped.

group "foo" {
  targets = ["foo"]
}
target "foo" {
  dockerfile = "bar"
}

$ docker buildx bake foo --print
{
  "group": {
    "default": {
      "targets": [
        "foo"
      ]
    }
  },
  "target": {}
}

Now:

$ docker buildx bake foo --print
{
  "group": {
    "default": {
      "targets": [
        "foo"
      ]
    }
  },
  "target": {
    "foo": {
      "context": ".",
      "dockerfile": "bar"
    }
  }
}

[errordeveloper]

I find JSON a bit nicer for these kind of tests, as you can indent each line as much as needed ;)

However, this test suite in particular has lots of strings with YAML, JSON & HCL... maybe a cleanup is due to move these assets to separate files.

[errordeveloper]

I think the group and target functions seem a bit hard to read, maybe less ambiguous names could help, or perhaps these could be rewritten a little. Just an observation...