[Issue 15086] import doesn't verify module declaration

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=15086

--- Comment #28 from Timothee Cour <timothee.cour2 at gmail.com> ---
Re-asking since no-one answered this:

Is there any single use case that current (broken) behavior allows that
couldn't be done with existing option such as -mv?
```
-mv=<package.module>=<filespec>  use <filespec> as source file for
<package.module>
```

If the answer is 'no but we don't want to break code that relied on this broken
behavior', could we at least start deprecation process for this or issue a
warning to use -mv instead?

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