Report only top-most deprecated module import

Fixes issue 13582

https://issues.dlang.org/show_bug.cgi?id=13582
> > current dmd front-end does not have enough information to trace import chain
> ? But isn't it exactly what this snippet does? `Module::scope` contains scope this module was originally imported from. Which has own `module` field and so on up to the root module.

It does not represent "import chain". Consider:

1) X imports A, and A imports B.
2) Y imports A, and A imports B.

Q: In that case, which X or Y would be pointed by `A->scope`?
A: it depends on which 1) or 2) will be analyzed first.

The true "import chain" from module A should refer both X and Y, but `Module::scope` doesn't. So there's no import chain in dmd front-end.