Doesn't work as a vendored library: interface mismatch on Metapp_preutils__

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Usually it's possible to use vendored libraries and ppxs using dune by simply copying them into a subdirectory. This seems to fail with metapp:

% cat test.ml 
let _ = [%meta [%e 42]]
% cat dune
(executable
 (name test)
 (preprocess (pps metapp.ppx)))
% dune build test.exe  
         ppx test.pp.ml
% opam source metapp
Successfully extracted to metapp.0.4.0
% dune build test.exe
         ppx test.pp.ml (exit 1)
(cd _build/default && .ppx/e7c203cacf1cd3bb02e6242329effa31/ppx.exe -o test.pp.ml --impl test.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
File "_none_", line 1:
Error: interface mismatch on Metapp_preutils__