internal/trace: rename go122 to tracev2

internal/trace: rename go122 to tracev2

This change follows up from the previous one which renamed oldtrace to
tracev1, defining everything Go 1.22+ as trace v2.

This change also re-maps some packages in preparation for sharing with
other parts of the standard library, like the runtime. It also cleans up
some other uses of 'go122' that are just a bit misleading. The mappings
are as follows:

- internal/trace/event -> internal/trace/tracev2/event
- internal/trace/event/go122 -> internal/trace/tracev2
- internal/trace/internal/testgen/go122 ->
  internal/trace/internal/testgen

The CL updates all import paths and runs gofmt -w -s on the entire
subdirectory.

Change-Id: I35476c679a96d4eafad6b94bac5f88aa7b085d2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/644218
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>