`pg`: Add `ScopeName` -> `Vec<GroupName>` mapping #427

Job	Run time
docs	2m 43s
Clippy	2m 31s
benches	3m 50s
rustfmt	28s
Run the default tests	3m 45s
Test ractor in async_std	1m 23s
Test ractor with the `cluster` feature	3m 45s
Test ractor_cluster	3m 23s
	21m 48s

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