fix: track a remote fragment URL per deployer file reference

What does this MR do?

Track the remote fragment URL per deployer file reference instead of globally, so each reference resolves to and retains its own URL.

Before and after

Before: A single remote fragment URL was tracked globally, so references from different deployer files could clobber one another and the wrong fragment could be resolved.

After: Each reference keeps its own URL, and a re-declaration without the reference or without a kustomize path keeps the originally recorded values.

How to validate locally

1. Run go test ./internal/config -run TestRecordElementSources_tracksURLPerFileReference.
2. Confirm all sub-cases pass.

Acceptance checklist

• Two files referencing different remote fragments keep distinct URLs
• A re-declaration without the reference keeps the original fragment
• A re-declaration without a kustomize path does not blank the recorded one
• Tests cover per-reference tracking