rls gets confused by stray `.merlin` files

See https://reasonml.chat/t/how-to-debug-reason-vscode-and-an-error-could-not-find-local-build-dir/1370

Currently it uses the presence of a `.merlin` file in a source directory as an indicator that we're dealing with a dune project, but we should probably ignore that b/c the presence of a `dune` file is the real indicator.

Start by locating the project-detection entry point that treats a .merlin file in a source directory as evidence of a dune project. Reproduce the stray-.merlin case described in the linked discussion, then make detection rely on the presence of a dune file and verify that genuine dune projects still work.

Written by the indexing model from the issue text.