spec: introduce alias declarations and type definitions

To avoid confusion caused by the term "named type" (which now just
means a type with a name, but formerly meant a type declared with
a non-alias type declaration), a type declaration now comes in two
forms: alias declarations and type definitions. Both declare a type
name, but type definitions also define new types.

Replace the use of "named type" with "defined type" elsewhere in
the spec.

For #18130.

Change-Id: I49f5ddacefce90354eb65ee5fbf10ba737221995
Reviewed-on: https://go-review.googlesource.com/36213


Reviewed-by: Rob Pike <r@golang.org>