I am not the expert in that code, but I don't think this is fixable, or in fact a bug.
go vet requires a program to successfully parse and typecheck before it starts running the actual vet checks, that error is from the load imports phase which is before vet runs at all. You will get the same error from go build

Agreed with @ianthehat. We could perhaps hack vet to run a subset of the checks (like the build tag one) before type-checking, but that would probably result in added complexity for little benefit.

I think fixing this would provide significant benefit. The issue came from inside my company where a developer spent a day trying to figure out what was wrong with their tools.go file--a cute hack that appears to have metastasised across Go codebases. My snooty response that go vet would catch this was undermind by the fact that even though this file is sufficient to drive the go mod mechanisms vet is too picky to spot the obvious error on line one; the line that would have helped it avoid spending time on the rest of the file.