cmd/go: when expanding "cmd", skip vendored main packages

cmd/go: when expanding "cmd", skip vendored main packages

We are vendoring pprof from github.com/google/pprof, which comes with
a main package. If we don't explicitly skip that main package, then
`go install cmd` will install the compiled program in $GOROOT/bin.

Fixes #19441.

Change-Id: Ib268ffd16d4be65f7d80e4f8d9dc6e71523a94de
Reviewed-on: https://go-review.googlesource.com/38007
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Raul Silvera <rsilvera@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>