[Issue 16673] improve cyclic module imports checker

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16673

--- Comment #2 from Steven Schveighoffer <schveiguy at yahoo.com> ---
I think Martin meant *data* symbols, not functions. Obviously function symbols
in local module can depend on external items. Of course, the compiler could
extend the check to the local function as well if possible.

I'd go further and say that depending on external compile-time constants can
also be allowed (e.g. enums).

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