opam install --deps-only ./<pkg>.opam modifies permanently the definition of <pkg> if it is already installed

[kit-ty-kate]

While testing a fix for #5567 i noticed that the following test:

opam pin remove -y test
printf 'opam-version: "2.0"\n' > ./test.opam
opam install -y ./test.opam
printf 'opam-version: "2.0"\ndepends: "core"\n' > ./test.opam
opam install --no ./test.opam --debug-level=3 --deps-only

would modify the pinned test package in a way where it now show core as part of the dependencies instead of what was there before.

I think it is relatively accepted that --deps-only should not do anything apart from installing the dependencies of a given package and thus this behaviour shouldn't happen

[kit-ty-kate]

Looking at the code, i suspect this also happens with opam upgrade --check