`pg`: Add `ScopeName` -> `Vec<GroupName>` mapping #408

Job	Run time
rustfmt	14s
Clippy	2m 5s
docs	2m 7s
benches	4m 3s
Run the default tests	3m 53s
Test ractor in async_std	3m 53s
Test ractor with the `cluster` feature	4m 3s
Test ractor_cluster	3m 35s
	23m 53s

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