cmd/compile: consistent unified IR handling of package unsafe

Within the unified IR export format, I was treating package unsafe as
a normal package, but expecting importers to correctly handle
deduplicating it against their respective representation of package
unsafe.

However, the surrounding importer logic differs slightly between
cmd/compile/internal/noder (which unified IR was originally
implemented against) and go/importer (which it was more recently
ported to). In particular, noder initializes its packages map as
`map[string]*types2.Package{"unsafe": types2.Unsafe}`, whereas
go/importer initializes it as just `make(map[string]*types.Package)`.

This CL makes them all consistent. In particular, it:

1. changes noder to initialize packages to an empty map to prevent
further latent issues from the discrepency,

2. adds the same special handling of package unsafe already present in
go/internal/gcimporter's unified IR reader to both of cmd/compile's
implementations, and

3. changes the unified IR writer to treat package unsafe as a builtin
package, to force that readers similarly handle it correctly.

Fixes #52623.

Change-Id: Ibbab9b0a1d2a52d4cc91b56c5df49deedf81295a
Reviewed-on: https://go-review.googlesource.com/c/go/+/403196
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions