Context

!3366 (merged) was merged. This MR is quality-of-life follow-up to:

ensure that if a new library is added, it must be declared in manifest/main.ml, either by declaring it as normal to generate its dune and .opam files, or to explicitely declare that we don't want to generate it;
add a .merlin, which is needed for merlin integration since we do not use dune to compile the manifest.

The first point was actually already implemented but as a warning only, which thus didn't cause the CI to fail.

Manually testing the MR

Try to use merlin in Emacs.

To test the error, do:

touch src/dune
make -C manifest
echo "exit code = $?"

You should see an error and an exit code of 1 (which ensures that the CI will fail).

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
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Edited Jan 07, 2022 by Romain