zip: allow packages containing non-top-level /vendor/ part

[gucio321]

This fixes https://golang.org/issues/71785

there was no clear statement in the above issue, but I was so frustrated after this not being fixed in 1.24, that I did a fix that you guys could just merge if it fits you or do a better one.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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