`pg`: Add `ScopeName` -> `Vec<GroupName>` mapping #406

Job	Run time
Clippy	2m 3s
docs	2m 6s
benches	3m 52s
rustfmt	11s
Run the default tests	4m 2s
Test ractor in async_std	4m 3s
Test ractor with the `cluster` feature	4m 1s
Test ractor_cluster	3m 53s
	24m 11s

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