cmd/go: add better logging for go mod vendor

[rittneje]

Today when we run go mod vendor, we only get a single message printed to stderr per dependency that looks like go: downloading golang.org/x/text v0.3.7.

This does not tell us when the package finished downloading, which is especially important when trying to diagnose why go mod vendor is slow. (Since go mod vendor seems to download lots of stuff in parallel, we cannot just go by the next log line.)

It also does not tell us which particular proxy in the GOPROXY list was used, which is again important for troubleshooting.

Please add the aforementioned logs, perhaps via some -verbose flag or similar.

[seankhliao]

related #26152 #39728

[ianling]

Sorry if this is too tangential for this issue, but is there a reason why this stuff all logs to stderr instead of stdout? These informational messages are not errors.

Same with the messages that come up after a go get run : https://github.com/golang/go/blob/master/src/cmd/go/internal/modget/get.go#L1692

[rittneje]

@ianling Typically, stdout is for the actual output of a command (which tends to be more stable for parsing, etc.), while stderr is used for logging (not just error messages). Not all commands abide by this though.