Omit function file in composition render when crossplane.yaml exists

### What problem are you facing?

Similarly as what happens when there's a project file found, we should be able to process a `crossplane.yaml` file too.

Not all deployments are based on Projects, but rely on configuration packages. For those cases, a preprocess of the `crossplane.yaml` file to extract the functions is usually required.

### How could Crossplane help solve your problem?

Treat `crossplane.yaml` file similarly as we do for the project file, and read the functions from there (thus, allow omit that file).