cmd/go: handle wildcards for unknown modules in "go get"

For example, "go get golang.org/x/tools/cmd/..." will add a
requirement for "golang.org/x/tools" to go.mod and will install
executables from the "cmd" subdirectory.

Fixes #29363

Change-Id: Id53f051710708d7760ffe831d4274fd54533d2b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/171138


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>