cmd/go: add PGO auto mode

cmd/go: add PGO auto mode

Add "auto" mode for the -pgo build flag. When -pgo=auto is
specified, if there is a default.pgo file in the directory of the
main package, it will be selected and used for the build.

Currently it requires exactly one main package when -pgo=auto is
specified. (We'll support multiple main packages in the future.)

Also apply to other build-related subcommands, "go install", "go
run", "go test", and "go list".

For #55022.

Change-Id: Iab7974ab8932daf0e83506de505e044a8e412466
Reviewed-on: https://go-review.googlesource.com/c/go/+/438737
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>