runtime: skip doInit of plugins in runtime.main

runtime: skip doInit of plugins in runtime.main

Plugins may be loaded in the user's init code.
If loading fails, md.bad is true, and doInit should not be executed.
If loading succeeds, the plugin must run modulesinit and typelinksinit
before doInit. Here is not protected by pluginsMu, and in concurrent
scenarios it is possible to obtain the moduledata of the plugin that
is still in the loading process.
Any added modules after loop starts will do their own doInit calls.
This fixes the issue introduced by CL 520375.

Fixes #75102

Change-Id: I48e91ae21615a0c54176875a6a2dea8e1dade906
Reviewed-on: https://go-review.googlesource.com/c/go/+/697675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>