[Issue 18517] Import order is not invariant

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18517

--- Comment #3 from Steven Schveighoffer <schveiguy at yahoo.com> ---
Hm... I didn't realize that the module identifier was not identical to the
filename. That is interesting. It's not clearly stated in this bug report that
this is a requirement to get the error to occur -- I thought it had something
to do with the fact that `thing` appears in both files.

I would expect that if a module is loaded via using the filesystem, and the
module name doesn't match how it was found, then this should be an error.
Anything else seems to strain your sanity!

In other words, if you pass the modules on the command line, then they can have
non-matching filenames, but if they were found via the module name mapping to
directories, they had better match the module name.

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