go/types: fix internal documentation and strengthen assumptions

Imported (incl. dot-imported) objects are always in file scope,
never in package scope. Fix misleading comment.

Package-scope declared objects must have objMap entry by
construction (of that map). Remove unnecessary check and
corresponding misleading comment.

Found while investigating solutions for @23203.

Change-Id: Idadfdd1576681ae56e11daa27bdcee2c73733c41
Reviewed-on: https://go-review.googlesource.com/110916


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>