… embedded types of imported interfaces

[This is a re-apply of CL 571075]

Imported interfaces don't have position information for embedded types.
When computing the type set of such interfaces, doing a version check
may fail because it will rely on the Go version of the current package.

We must not do a version check for features of types from imported
packages - those types have already been typechecked and are "correct".
The version check code does look at packages to avoid such incorrect
version checks, but we don't have the package information available
in an interface type (divorced from its object).

Instead, for now rely on the fact that imported interfaces don't have
position information for embedded types: if the position is unknown,
don't do a version check.

We may want to assert that positions are known in all other cases,
but since this is an older release, don't add such additional changes
to avoid introducing other bugs.

Fixes #66326.
Updates #66064.

Change-Id: I158cf51aa382f85d612ab958ba4b591de1c5fdb2
Reviewed-on: https://go-review.googlesource.com/c/go/+/574736
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>