Report only top-most deprecated module import

Fixes issue 13582

https://issues.dlang.org/show_bug.cgi?id=13582
As far as I know, current dmd front-end does not have enough information to trace import chain. So I cannot understand what this code does.

And why you need to trace the import chain? I think the rule can be simplified as:

1. Non-deprecated module A imports non-deprecated module B, no message printed.
1. Non-deprecated module A imports deprecated module B, deprecation massage is printed.
1. Deprecated module A imports non-deprecated module B, no message printed.
1. Deprecated module A imports Deprecated module B, no message printed.