cmd/go: provide a convenient way to iterate all packages in go.work workspace

[hyangah]

go.work makes it easier to work with multiple modules.

Often users want to run tests or analysis for all packages in their workspace. For a single module case, this can be easily achieved by using ./... pattern. Currently, however, there is no equivalent of ./... in workspace mode. So users need to run go commands from inside of each individual module, or supply the paths for each module directories listed in go.work. (e.g. go test -v $(go list -f '{{.Dir}}/...' -m | xargs)).

We need to figure out how to offer ./...-equivalent behavior when working with `go.work.

My personal preference is to reuse./... (VSCode Go uses ./... pattern to implement workspace-wide test, lint, coverage computation, etc).

p.s. I noticed the issue about clarifying the meaning of ... in module mode is still open #37227