cmd/compile: remove skipping of implicit operations during export

We'll need to attach types to these operations, so we need to
represent them in the import/export data.

Some of the operations use a selector indicating a different package,
so we need to provide an option to encode the package of a selector.
The default selector() function can't encode that extra information,
as selector's exact encoding is used by go/types.

Change-Id: I4c110fe347b3d915f88a722834bc4058baea7854
Reviewed-on: https://go-review.googlesource.com/c/go/+/299771


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>