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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This PR (HEAD: 2b5b64f) has been imported to Gerrit for code review.

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