(subdir ...) stanzas do not work in included dune files

[nojb]

To reproduce:

$ cat >dune-project <<EOF
> (lang dune 2.5)
> EOF
$ cat >dune <<EOF
> (include dune.inc)
> EOF
$ cat >dune.inc <<EOF
> (subdir a (rule (alias foo) (action (echo Hello))))
> EOF
$ mkdir -p a
$ dune build @foo
File "dune.inc", line 1, characters 2-8:
1 |  (subdir a (rule (alias foo) (action (echo Hello))))
      ^^^^^^
Error: Unknown constructor subdir

[nojb]

Looking at how subdir is implemented, I understand that allowing this would complicate matters, since the subdir stanzas are interpreted before evaluating any rule.

On the other hand, this feature is useful in generated dune files specifically, so it would nice to support this if not too difficult.

[nojb]

One way to do it could be to process (include ...) stanzas before (or at the same time as) (subdir ...).

[ghost]

That seems reasonable

[rgrinberg]

It works now.

[vsiles]

@rgrinberg FYI it doesn't work easier with tuareg mode files like https://github.com/facebook/hhvm/blob/master/hphp/hack/src/hhi/dune