Presence of .merlin.skip-if-not-cwd skips config in dir

[jonahbeckford]

Mitigation for #1869

414 branch: https://github.com/jonahbeckford/merlin/tree/feature-skip-if-not-cwd-414

Testing (see Reproduction in #1869):

# Base case has a failure because there is a us\SonicScoutBackend\dune-project
PS> ... ocamlmerlin.exe single dump-configuration -filename us\SonicScoutBackend\src\SonicScout_Std\Aws_manager.ml | jq '.value.merlin.failures, (.value.merlin.build_path | length)'
[
  "No config found for file us/SonicScoutBackend/src/SonicScout_Std/Aws_manager.ml. Try calling 'dune build'."
]
0

# Adding .merlin.skip-if-not-cwd to subproject SKIPS .merlin and dune-project
PS> echo "" > .\us\SonicScoutBackend\.merlin.skip-if-not-cwd
PS> ... ocamlmerlin.exe single dump-configuration -filename us\SonicScoutBackend\src\SonicScout_Std\Aws_manager.ml | jq '.value.merlin.failures, (.value.merlin.build_path | length)'
[]
227

# Adding .merlin.skip-if-not-cwd to root project DOES NOT SKIP .merlin or dune-project
PS> echo "" > .merlin.skip-if-not-cwd
PS> ... ocamlmerlin.exe single dump-configuration -filename us\SonicScoutBackend\src\SonicScout_Std\Aws_manager.ml | jq '.value.merlin.failures, (.value.merlin.build_path | length)'
[]
227