Support `textDocument/definition` when reusing a single attribute from another target block

[rcjsuen]

It is possible to reuse attributes from another Bake target. We should be able to get a valid response for a textDocument/definition request when the text cursor is around target.foo or target.foo.tags.

target "foo" {
  dockerfile = "foo.Dockerfile"
  tags       = ["myapp:latest"]
}

target "bar" {
  dockerfile = "bar.Dockerfile"
  tags       = target.foo.tags
}

{
  "group": {
    "default": {
      "targets": [
        "bar"
      ]
    }
  },
  "target": {
    "bar": {
      "context": ".",
      "dockerfile": "bar.Dockerfile",
      "tags": [
        "myapp:latest"
      ]
    }
  }
}