spec: consistently use "defined type" and "type name" (cleanup)

When we introduced the notion of alias type declarations, we renamed
"named type" to "defined type" to avoid confusion with types denoted
by aliases and thus are also types with names, or "named types".

Some of the old uses of "named types" remained; this change removes
them.

Now the spec consistently uses the terms:

- "defined type"  for a type declared via a type definition
- "type name"     for any name denoting an (alias or defined) type
- "alias"         for a type name declared in an alias declaration

New prose is encouraged to avoid the term "named type" to counter-
act further confusion.

Fixes #23474.

Change-Id: I5fb59f1208baf958da79cf51ed3eb1411cd18e03
Reviewed-on: https://go-review.googlesource.com/89115


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