Negate Golang 1.16 go mod download affecting go.sum

[ericvn]

Golang 1.16 RC 1 has a behavior change where go mod download affects the go.sum file adding additional listed modules. These additional modules in go.sum are removed with a go mod tidy. @howardjohn wrote golang/go#43994 for the change in behavior which was closed as working as designed.

Two Istio PRs were already merged to change the order of targets so that the go mod tidy happened after the go mod download during a make gen so the go.sum file doesn't highlight a change. This PR is adding a go mod tidy within the makefile target containing the go mod download so the target itself won't have an overall effect on go.sum.

Some testing shows that go mod download places about 1.55 GB of modules into the cache. In John's PR they mentioned using go list instead of go mod download as the number of modules downloaded will be less and go.sum will not be affected. go list downloads about 750 MB worth of files.

The go mod download was added into the makefiles when the license-lint related targets were added. It was a precursor to the license-lint call. The license-lint code actually does a go list -mod=readonly -deps -test ./... so doing the go mod download is not actually needed.

I did add an extra target which downloads the files using go list, just to highlight an alternative command/target. It is possible to remove the lines between the mod-download-go and list-all-go targets so both just do the go list.

@jwendell, I know you made some changes in this space, so would like your feedback as well.

[howardjohn]

should we just remove the download depency for the license target as well?