Validate if a file with the same blake2 digest already exists (#2490)

[yeraydiazdiaz]

Finishing off @alanbato's PR to fix #2490. Closes #2928.

Introduces a subtle shift in how we consider a file to exist in the database to be either the filename or the blake2 hash, which I believe should be safe enough.

Thanks for starting work on this one @alanbato 👍

[ewdurbin]

@yeraydiazdiaz thanks for picking this up!

Can we update the error message at https://github.com/yeraydiazdiaz/warehouse/blob/be94c82324c18c9e7c12a0c79d83cacac1a2cd58/warehouse/forklift/legacy.py#L1083-L1088 and help-text at https://github.com/yeraydiazdiaz/warehouse/blob/be94c82324c18c9e7c12a0c79d83cacac1a2cd58/warehouse/templates/pages/help.html#L225-L238 to note the new possible reasons for the error?

[di]

Yeah, I agree with @ewdurbin, either we need to make this a separate error that is explicit about the file being rejected b/c the hash matches, or we need to update the existing help text (this happens rarely enough that the latter should suffice).

[di]

Thanks @yeraydiazdiaz!

[alanbato]

Hooray @yeraydiazdiaz! Thanks for finishing it up right and proper :)