cmd/cgo: move typedefs and typedefList out of Package

cmd/cgo: move typedefs and typedefList out of Package

Theyre moved into a new fileTypedefs type so we can better keep track of
their lifetime.

For #75167

Change-Id: I6a6a696491d00eb4b1cc56dfcb9e94ed53573a7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/699015
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>