Support cyclic imports and zero-forward-declaration semantics

[lbcheng888]

Summary

• support cyclic imports and zero-forward-declaration semantics in the compiler
• make the user-module path work without manual forward declarations while keeping stdlib/system on the conservative path
• async ORC leak fixes moved to stdlib: fix async ORC resource leaks #25683

Testing

Build a test compiler from compiler/nim.nim:

./bin/nim c --skipUserCfg --skipParentCfg -d:nimKochBootstrap -o:bin/nim_default_reorder compiler/nim.nim

Run the regression checks with that compiler:

./bin/nim_default_reorder c -r tests/modules/tcyclicimports_proc.nim
./bin/nim_default_reorder c -r tests/modules/tcyclicimports_types.nim
./bin/nim_default_reorder c -r tests/modules/tcyclicimports_noreorder.nim
./bin/nim_default_reorder c -r tests/misc/tnoforward.nim
./bin/nim_default_reorder c -d:testdef -r tests/modules/treorder.nim
./bin/nim_default_reorder c -r tests/pragmas/treorder.nim
./bin/nim_default_reorder c -r tests/overflow/toverflow_reorder.nim --overflowChecks:off
./bin/nim_default_reorder c tests/modules/tselfimport.nim

[Araq]

Make "fix ORC-related async resource leaks in asyncdispatch, the kqueue selector backend, and Windows AsyncEvent handling" its own PR.