[Issue 23193] druntime/win32.mak, win64.mak, posix.mak contain no instructions for how to build druntime

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23193

--- Comment #6 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Walter Bright from comment #5)
> if not exist generated md generated
> if not exist generated\windows md generated\windows
> if exist "../../release-build/dmd-2.079.0/windows/bin/dmd.exe" (echo
> @"../../release-build/dmd-2.079.0/windows
> /bin/dmd.exe" %* >generated\windows\host_dmd.bat)
> Error: command line too long
> 
> --- errorlevel 1
> 
> --- errorlevel 1
> ----------
So really, this is the problem.  We've now so many files in druntime, that
we've exceeded the command-line limit?

https://github.com/dlang/druntime/pull/3026

Perhaps this copyimports.d helper should be adapted to just read/parse in
`mak/COPY` directly and do the work, rather than us giving it the list of
files.

On a related note, I can't see why we still have generated .di headers in
druntime, so opened https://github.com/dlang/druntime/pull/3850 to remove them,
and converge everything into the `make copy` list.

That'll avoid some of the confusion here regarding `make import` doing
something different to what might be expected.

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