Fix handling of filename for keywords

[bwestlin]

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Description of Changes

Fix filename for generated .rs file when it is names as a keyword. For example if any of the protos here https://github.com/googleapis/googleapis/tree/master/google/type is used the resulting file will be src/proto_types/google/r#type.rs which is not valid. After the changes here the file will instead be: src/proto_types/google/type.rs