utils/pypi: don't overwrite name/extras/version from basic_metadata if already set

[dtrodrigues]

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When any of @name, @extras, or @version aren't set, all are attempted to be inferred from the original URL. This change only updates the variables if they weren't already set.

Resolves #15653

[carlocab]

Looks reasonable to me!

[MikeMcQuaid]

Makes sense to me too, will get @woodruffw to give final confirmation!

[woodruffw]

LGTM, this was roughly the patch I had in mind. Thank you @dtrodrigues!