cmd/go: support multiple main packages with -pgo=auto #58099
Description
In Go 1.20, go build -pgo=auto can only be used with a single main package. Specifying multiple main packages (e.g., go build -pgo=auto cmd/compile cmd/link) is an error.
To support this, cmd/go needs to learn to build the transitive dependencies N times, once for each different profile used by each main package.
Follow up to #55022.