cmd/go: support multiple main packages with -pgo=auto

[prattmic]

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.

cc @cherrymui @aclements

[nemo-cpt]

I hope everone here understands that this N-fold complete rebuild doesn't scale.

Having 100s of services and 1000s of packages you end up in extremely large caches (100x than normal) or basically dong every build without cache, or mix of both.

[gopherbot]

Change https://go.dev/cl/472357 mentions this issue: cmd/go: make PGO profile path per package

[gopherbot]

Change https://go.dev/cl/472358 mentions this issue: cmd/go: support multiple main packages with -pgo=auto

[gopherbot]

Change https://go.dev/cl/473275 mentions this issue: cmd/go: distinguish packages built for different main packages in printing