glob_file_rec is not recursive on generated folder

Expected Behavior

When using glob_files_rec inside a generated dir, I would expect the glob to be recursive.

Actual Behavior

It behaves like a non recursive glob.

Reproduction

• PR with a reproducing test: #9774

Additional information

I think this is done on purpose, glob_files_rec.t has some sort of tests for that already. I think this either should not apply to target dirs built with an explicit rule, or should be disable if the generated dir is in the prefix of the glob.

That is :

  $ cat > dune <<EOF
  > (library
  >  (name foo))
  > 
  > (rule
  >  (alias x)
  >  (deps (glob_files_rec *.cmi))
  >  (action (bash "for i in %{deps}; do echo \$i; done")))
  > EOF

Would return nothing, but

  $ cat > dune <<EOF
  > (library
  >  (name foo))
  > 
  > (rule
  >  (alias x)
  >  (deps (glob_files_rec foo/*.cmi))
  >  (action (bash "for i in %{deps}; do echo \$i; done")))
  > EOF

would work recursively on subdirs of foo.

Start with the reproducing test in PR #9774 and the existing tests in glob_files_rec.t. Compare recursive glob behavior when the generated directory is implicit versus explicitly built. Done means resolving the generated-directory case without regressing the existing glob_files_rec.t behavior.