WIP: fix directory targets with empty subdirs #29335

Job	Run time
Format	37s
Nix (macos-latest)	1m 14s
Nix (ubuntu-latest)	29s
Build microbenchmarks	1m 13s
Build (macos-latest, 5.1.x)	1s
Coq 8.16.1	1m 12s
Build (ubuntu-latest, 5.1.x)	1m 0s
Create local opam switch (macos-latest, 5)	1m 13s
Documentation	1m 1s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	58s
Create local opam switch (macos-latest, 4.14)	58s
Build monorepo benchmark docker image	1m 9s
Build (4.14.x, ubuntu-latest, true)	58s
Create local opam switch (ubuntu-latest, 5)	1m 12s
Build (4.14.x, macos-latest, true)	59s
Create local opam switch (ubuntu-latest, 4.14)	1m 11s
Wasm_of_ocaml	1m 11s
Build (5.1.x, macos-13, true)	59s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	57s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	56s
Build (4.08.x, ubuntu-latest, true)	59s
Build (4.04.x, ubuntu-latest, true, true)	58s
Build (4.02.x, ubuntu-latest, true)	1m 12s
	22m 37s

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