Detect improper YAML keys

[dmlloyd]

Description

Some Quarkus-like systems, when configuring with YAML, non-conformingly treat foo.bar: the same as foo:\n bar:. If someone erroneously passes in a key of the form quarkus.foo: in a YAML file, we ignore it silently, which might be very confusing to the user.

If we detect any configuration key segment starting with quarkus., we should throw an invalid key error. This would also detect erroneously giving "quarkus.foo"=something in an application.properties file.

Implementation ideas

No response