cmd/go: implement 'go install pkg@version'

With this change, 'go install' will install executables in module mode
without using or modifying the module in the current directory, if
there is one.

For #40276

Change-Id: I922e71719b3a4e0c779ce7a30429355fc29930bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/254365


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>