proposal: cmd/go: add -maxdepth to go work use -r

[sding3]

I propose that we add a new optional -maxdepth <uint> flag to go work use -r moddirs. It limits the level of directories to descend into for each moddirs, like how the -maxdepth in the Unix find command works.

[seankhliao]

what are the practical/real world cases is which this is useful?

[sding3]

most go modules are defined at the root directory of repos, and most people have go projects cloned onto their local filesystem either in one directory (flat layout) or in a hierarchy of sub-directories with a known/limited depth. So a -maxdepth flag can be used to limit the search scope in these cases and prevent bogus or unwanted go.mod files from becoming accidentally included in the work space (e.g. issue #51959) at which point users would then have to prune those out, which is cumbersome.

[seankhliao]

Depth seems like a poor proxy for the actual issues, a multi module repository can easily have modules at different depths, while a checked out dependency in the same workspace (eg, x/tools from the other issue) may have "unwanted" go.mods at the same depths.
I don't see this as a generally useful mechanism for excluding unwanted module declarations.

[sding3]

-maxdepth is only useful for cases where the desirable depth of directory for where to find go modules to be included in the work space is known. I suspect this is the common case.

Even for cases where a directory level contains both desirable go modules and undesirable ones, limiting the search scope with a max depth would result in fewer (or equal) undesirable ones that need to be pruned out than that of allowing the search scope to be unbound.

[ianlancetaylor]

CC @bcmills @matloob