nixd/Server: construct names of nested attr set #1466

Job	Run time
nix-build-develop-gcc (none, release, true)	4m 45s
nix-build-develop-gcc (none, release, false)	6m 32s
nix-build-develop-gcc (none, debug, true)	6m 3s
nix-build-develop-gcc (none, debug, false)	4m 34s
nix-build-develop-gcc (undefined, release, true)	12m 13s
nix-build-develop-gcc (undefined, release, false)	10m 52s
nix-build-develop-gcc (undefined, debug, true)	5m 19s
nix-build-develop-gcc (undefined, debug, false)	7m 41s
nix-build-develop-clang (none, release, true)	5m 0s
nix-build-develop-clang (none, release, false)	4m 4s
nix-build-develop-clang (none, debug, true)	3m 46s
nix-build-develop-clang (none, debug, false)	10m 28s
	1h 21m 17s

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