cmd/go: return an early error from queryImport when in vendor mode

[rohnnyjoy]

The current behavior for -mod=vendor is to let QueryPackages run and
fail from queryImport: "cannot query module due to -mod=vendor".
This has the side effect of allowing "go: finding module for package"
to be printed to stderr. Instead of this, return an error before
running QueryPackages.

Fixes #58417

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Patch Set 1: Code-Review+2 Run-TryBot+1

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Patch Set 2:

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This PR is being closed because golang.org/cl/467517 has been merged.