fix: allow multiple (dirs ..) stanzas

[anmonteiro]

fixes #6249

[rgrinberg]

I don't think we should making the evaluation of stanzas in a dune file dependant on the order.

[anmonteiro]

thank you, i agree. do you have a suggestion on how we'd handle something like:

(dirs foo)
(dirs :standard \ foo)
(dirs foo)