.merlin based projects still require a blank dune-project file

[gdvolkov-isp]

Thanks for adding the --fallback-read-dot-merlin option in #705!

It seemed to do nothing though, until I've looked at the code and realized why:

• config calls get_external_config and if that finds nothing, uses an initial blank config, i.e. doesn't get to reading .merlin
• get_external_config calls find_project_context
• find_project_context looks for a parent directory with dune-project or dune-workspace!

So currently it is necessary to touch dune-project to make a legacy .merlin non-dune project work. It would be nice if that wasn't the case.

[ulugbekna]

@anmonteiro hi! do you have opinion or would like to work on this since you authored #705 ?