[Lens] Fix bug in field list when _source contained fields with dots

[wylieconlon]

To reproduce this bug, you can create an index with a mapping such as:

PUT my_index
{
  "mappings": {
    "properties": { 
      "system": { 
        "properties": {
          "cpu": {
            "properties": {
              "pct": { "type": "float" }
            }
          }
        }
      }
    }
  }
}

And then index a document:

POST my_index/_doc/1
{
  "system.cpu.pct": 12.40
}

And then create an index pattern in Kibana, which you visit in Lens. It will show the field on the left:

Fixes #63630

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: a170be0

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[mbondyra]

Bug reproduced on master and tested and confirmed fixed here.