Skip "go: downloading example.com/abc/def/v1 v1.2.3" lines

[Infinoid]

If go vet is run prior to go test, it may be the first time the tests are parsed and the first time certain libraries are imported.
In this case, go vet outputs messages like this:

go: downloading mvdan.cc/xurls/v2 v2.2.0
go: downloading github.com/davecgh/go-spew v1.1.1
go: downloading github.com/pmezard/go-difflib v1.0.0
go: downloading gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b
go: downloading github.com/google/uuid v1.1.1

This isn't actually a problem. The libraries are in go.mod and go.sum already, they just weren't downloaded locally.

But the messages cause go-review to fail with:

could not parse line: 1, failed split location to filename and position

This PR skips those messages.

It is desirable to run go vet prior to go test. go test includes a subset of go vet's checks internally, but its output can't be piped to go-review, so those failures are not made visible in gerrit.

I am working around this for now by running go mod download beforehand. But it would be nice if go-review could just ignore them.