$ref containing external filename

[jayvdb]

e.g. input

components:
  schemas:
    CartesianCoordinate:
      $ref: "./schemas/CartesianCoordinate.yaml"

Most Rust openAPI readers have this problem, but https://github.com/kstasik/schema-tools has solved it, and has a CLI command to convert such an OpenAPI into a single YAML file without external references.