cmd/go: add -e flag to 'go get -u'

[josharian]

A missing dependency currently blocks go get -u from working at all. For a concrete example, see #28424 (comment).

go mod tidy and go mod vendor have a -e flag that means: "attempt to proceed despite errors
encountered while loading packages". I propose that we add an analogous -e flag to go get -u.

cc @bcmills

[bcmills]

This would make it easier to start an upgrade by running go get -e -u ./... and then fixing up the build failures through subsequent code changes (such as adjusting import statements within the main module). That seems like a completely reasonable workflow to me.

We do already have some logic in module-mode go get that attempts to suppress errors for transitive imports for as long as possible:
https://cs.opensource.google/go/go/+/master:src/cmd/go/internal/modget/get.go;l=1072-1084;drc=19309779ac5e2f5a2fd3cbb34421dafb2855ac21

So I think this would probably boil down to suppressing a couple of calls to base.SetExitStatus very late in the go get command:
https://cs.opensource.google/go/go/+/master:src/cmd/go/internal/modget/get.go;l=1476-1494;drc=19309779ac5e2f5a2fd3cbb34421dafb2855ac21

For such a low cost, this seems worth the usability benefit.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group