cmd/compile/internal/types: add Pkg and SetPkg methods to Type

The go/types API exposes what package objects were declared in, which
includes struct fields, interface methods, and function parameters.

The compiler implicitly tracks these for non-exported identifiers
(through the Sym's associated Pkg), but exported identifiers always
use localpkg. To simplify identifying this, add an explicit package
field to struct, interface, and function types.

Change-Id: I6adc5dc653e78f058714259845fb3077066eec82
Reviewed-on: https://go-review.googlesource.com/107622


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>