Clarify access to requestData properties

[matthew-white]

requestData classes have a number of internal properties. However, I don't think it's as clear as it could be that these properties are internal. This came up recently at #1024 (comment). I tried to make the properties private (that would be really clear), but that didn't work for a couple of reasons. Instead, I added a comment explaining our use of Symbol properties.

I also renamed the _name property to resourceName. I think it was particularly unclear whether that property was internal. Its name starts with _, but it's not a Symbol, and it's also intended to be used outside the file it's defined in. (Or if not used, at least included in console.log() output.) I think it doesn't hurt to remove the underscore and make it explicit how the property is meant to be used and that it's OK to use the property outside the file.

What has been done to verify that this works as intended?

Tests continue to pass.

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