reference values from overlay subsets

**Describe the problem/challenge you have**
I'm trying to set a label on multiple resources but with a dynamic value that includes the resource name.

```yaml
#@ load("@ytt:overlay", "overlay")

#@overlay/match expects="0+", by=overlay.subset({"kind": "Deployment"})
---
metadata:
#@overlay/match missing_ok=True
labels:
#@overlay/match missing_ok=True
foo: -bar #! <---------- dynamically set metadata.name of the same matching resource
```

**Describe the solution you'd like**
I want to be able to reference a value from overlay subsets.

**Anything else you would like to add:**

---
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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by tracing overlay.subset and overlay/match behavior in ytt's implementation and existing tests. Determine how a matched Deployment can reference its own metadata.name, then add focused coverage showing the expected label value for multiple matched resources. Done means the requested per-resource reference works without breaking existing overlay matching behavior.

Written by the indexing model from the issue text.