Crash with directory targets and multiple workspace contexts

[samoht]

See https://github.com/samoht/dune-crash for a repro.

Full error message:

Internal error, please report upstream including the contents of _build/log.
Description:
  ("gen_rules returned a set of directory targets that doesn't match the set of directory targets from returned rules",
  { dir = In_build_dir "solo5"
  ; mismatched_directories =
      map { "solo5/mirage" : { message = "not generated"; loc = "dune:1" } }
  })
Raised at Stdune__Code_error.raise in file
  "otherlibs/stdune/src/code_error.ml", line 10, characters 30-62
Called from Dune_engine__Load_rules.Load_rules.load_build_directory_exn.(fun)
  in file "src/dune_engine/load_rules.ml", line 850, characters 6-145
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
  line 76, characters 8-11
-> required by ("load-dir", In_build_dir "solo5")
-> required by
   ("build-alias", { dir = In_build_dir "solo5"; name = "default" })
-> required by ("toplevel", ())

I must not crash.  Uncertainty is the mind-killer. Exceptions are the
little-death that brings total obliteration.  I will fully express my cases.
Execution will pass over me and through me.  And when it has gone past, I
will unwind the stack along its path.  Where the cases are handled there will
be nothing.  Only I will remain.

[gridbugs]

I've simplified this repro and added a test that reproduces it #10330

[gridbugs]

It looks like this crash happens whenever a directory target is disabled via enabled_if. E.g.

; dune
(rule
 (enabled_if false)
 (target (dir x))
 (action (progn)))

; dune-project
(lang dune 3.14)
(using directory-targets 0.1)

[jchavarri]

See #10367 (comment).