Embedded interfaces in interfaces must take the form of a (possibly
qualified) type name. Before alias types, a type name always denoted
a defined (formerly "named") type. The introduction of alias types
enabled embedding of non-defined types via alias type names, as in:

type T interface { E }
type E interface { m() }

Both cmd/compile and gccgo accept this kind of code, and the spec does
not prohibit it. There may be code in the wild that makes use of this.

go/types was written under the assumption that embedded interfaces
were always defined types; and that assumption was even reflected in
the go/types API.

This change removes this restriction in the implementation (which
happens to make it simpler), and in the API (by adding additional
functions and deprecating the corresponding older versions).

It also replaces uses of NewInterface and Embedded (old API) by
NewInterface2 and EmbeddedType (new API) in dependent packages
(importers).

The old API remains in place for backward compatibility and is marked
as deprecated.

Fixes #25301.

Change-Id: I272acd498754179efaf0590ca49d3eb4eee4348e
Reviewed-on: https://go-review.googlesource.com/114317Reviewed-by: Alan Donovan <adonovan@google.com>