Remember to include CLI Go source files in `go mod vendor` cache

[mcandre]

Please don't strip cmd paths from go mod vendor caches. Altering the source cache makes it that much harder to track tool dependencies and debug Go systems generally.

[bcmills]

Per https://golang.org/cmd/go/#hdr-Make_vendored_copy_of_dependencies:

Vendor resets the main module's vendor directory to include all packages needed to build and test all the main module's packages.

CLI main packages are not importable, and thus are not “needed to build and test all the main module's packages” (in the sense of go build or go test). Therefore, they are omitted by design.

[bcmills]

Duplicate of #29516