cmd/go: when module enabled, `go clean` removes built binary

Now "go clean" can remove binary as expected, when module enabled and the module name isn't  "main" or the name of folder.

Fixes issue #41656

Change-Id: I54b9435ece045e03a12dc230efe84c8dd381a07c
GitHub-Last-Rev: f4ea2d8c765a6e3a2b89fe2c9fb5b2c80551efa5
GitHub-Pull-Request: golang/go#41999
Reviewed-on: https://go-review.googlesource.com/c/go/+/262677


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