Packages of dot imports don't appear in the Info.Implicits map
since they are already taken care of by the Info.Defs map. Fix
documentation.

Implicitly dot-imported objects of a package shouldn't appear
in the Info.Implicits map because the documentation never said
so and there's no way to map multiple objects to the same
*ast.ImportSpec with the current data structure.

Added missing test for Info.Implicits.

The fix is a trivial one-line deletion, the rest is documentation
and test.

Fixes #21591.

Change-Id: I12a37dab85c531911c9363ec3d58daa095c7eb24
Reviewed-on: https://go-review.googlesource.com/60672Reviewed-by: Alan Donovan <adonovan@google.com>