config: Remove confusing magic in path handling
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When passed a .yaml path (or any path with a non-whitelisted extension), don't read .yml instead when the .yaml exists. It's an extremely surprising behavior.
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If the .yaml file does not exist, it will still try alternative extensions in order not to break existing deployments which may rely on it, but it raises a warning now.
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When given a non-existing path, show an error log, but keep parsing it as an empty config, in order not to break existing deployments.