[dev.regabi] cmd/compile: separate noder more cleanly

Separate embed, cgo pragmas, and Main trackScopes variable
from noder more cleanly.

This lets us split embed and noder into new packages.
It also assumes that the local embedded variables will be
removed and deletes them now for simplicity.

Change-Id: I9638bcc2c5f0e76440de056c6285b6aa2f73a00d
Reviewed-on: https://go-review.googlesource.com/c/go/+/279299


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>