cmd/go: remove -d flag from all 'go get' hints in error messages

[jayconrod]

In Go 1.16, the go command prints error messages containing suggested commands to fix missing imports.

$ go list example.com/missing
no required module provides package example.com/missing; try 'go get -d example.com/missing' to add it

We should remove the -d flag from these messages. The -d flag is not essential to the operation, its meaning is not obvious to many people, and we're planning to deprecate it and make it a no-op in a future release.

[gopherbot]

Change https://golang.org/cl/282121 mentions this issue: cmd/go: make hints in error messages more consistent