cmd/go: add a `go work use` command

[matloob]

This proposal is an amendment of the Go Workspace Proposal (#45713) originally suggested by @jayconrod in a comment on that issue:

I'd like a way to add all modules in a subtree to a workspace using wildcards. Specifically, I'd like go workspace add ./....

• go workspace use would be a better subcommand name if we end up using that in go.work.
• I think this would help build a more familiar environment for folks used to working on related projects in GOPATH. I could put a go.work file in $GOPATH/src, then just have one VSCode window for all projects.
• We've talked about supporting wildcards in go mod initwork and go mod editwork, but that breaks some parallelism with go mod init, which only creates an empty go.mod file, and go mod edit, which only performs low-level syntactic edits without extra processing.

This proposal would add a new go command subcommand, perhaps called go mod addwork (if #48256 is accepted, the new subcommand would be called go work add or go work use).

It would ensure that the go.work command contains modules denoted by the given pattern, removing any matches of the pattern that no longer exist on disk.

cc @bcmills @ianthehat

[bcmills]

Would there be a corresponding command to remove modules from the workspace? (That could be useful for, say, scripting.)

[matloob]

Modules can still be removed by go mod editwork, and go mod addwork will remove any modules that match the pattern but don't exist on disk.

[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

[hyangah]

@rsc Is it possible to review #48256 together at the proposal review meetings? (go mod addwork vs go work add) That proposal is not in the active column yet.

This proposal would add a new go command subcommand, perhaps called go mod addwork (if #48256 is accepted, the new subcommand would be called go work add or go work use).

[rsc]

@hyangah added to my list for next meeting, thanks. Not sure how I missed that one.

[matloob]

In consultation with @jayconrod @rsc and @bcmills, we want to rename this proposed command to go work use (or go mod usework if #48256 isn't accepted). Along with renaming the command, we'd rename the directory to to use. go work use would add a single directory or set of directories stated on the command line and go work use -r would search all subdirectories recursively and add the modules it finds (or remove modules in the go.work file that are contained in the directory but no longer exist).

[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