## Description 

Previously our map is represented in the server by a flattened
`(address, symbol)` key for looking up sources.

```json
{ "mainnet" : { "address, symbol" : "source" } }
```

Now the lookup becomes:

```json
{ "mainnet" : { "address" : { "symbol" : "source" } } }
```

This proper nesting simplifies / deduplicates printable representations
of our sources map for clients to consume.


## Test Plan 

Mechanical change covered by existing tests.

---
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### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes