new build issue of salsa-core (opam 2.5 or topkg related?)

[hannesm]

I spotted in https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/1fe451573794cb4fbfc1744e7addec18c91d3810/variant/compilers,4.14,mirage-runtime.4.10.4,revdeps,salsa20-core.0.1.0 (PR #28942) the following issue:

#=== ERROR while compiling salsa20-core.0.1.0 =================================#
# context              2.5.0~beta1 | linux/x86_64 | ocaml-base-compiler.4.14.2 | file:///home/opam/opam-repository
# path                 ~/.opam/4.14/.opam-switch/build/salsa20-core.0.1.0
# command              ~/.opam/opam-init/hooks/sandbox.sh build ocaml pkg/pkg.ml build --pinned false
# exit-code            1
# env-file             ~/.opam/log/salsa20-core-7-244a4b.env
# output-file          ~/.opam/log/salsa20-core-7-244a4b.out
### output ###
# + ocamlfind ocamlc -config
# + ocamlfind ocamldep -package cstruct -package nocrypto -modules salsa20_core.ml > salsa20_core.ml.depends
# + ocamlfind ocamldep -package cstruct -package nocrypto -modules salsa20_core.mli > salsa20_core.mli.depends
# + ocamlfind ocamlc -c -g -annot -bin-annot -package cstruct -package nocrypto -w +A-4-6-7-9-40-42-44-48 -warn-error +1..49 -o salsa20_core.cmi salsa20_core.mli
# + ocamlfind ocamlopt -c -g -annot -bin-annot -package cstruct -package nocrypto -w +A-4-6-7-9-40-42-44-48 -warn-error +1..49 -o salsa20_core.cmx salsa20_core.ml
# + ocamlfind ocamlopt -a salsa20_core.cmx -o salsa20-core.cmxa
# + ocamlfind ocamlc -c -g -annot -bin-annot -package cstruct -package nocrypto -w +A-4-6-7-9-40-42-44-48 -warn-error +1..49 -o salsa20_core.cmo salsa20_core.ml
# + ocamlfind ocamlopt -shared -linkall salsa20-core.cmxa -o salsa20-core.cmxs
# + ocamlfind ocamlc -a salsa20_core.cmo -o salsa20-core.cma
# Solver failed:
#   Ocamlbuild knows of no rules that apply to a target named ./salsa20_core.mly. This can happen if you ask Ocamlbuild to build a target with the wrong extension (e.g. .opt instead of .native) or if the source files live in directories that have not been specified as include directories.
# Backtrace:
#   - Failed to build the target pkg.otarget
#       - Building pkg.otarget:
#           - Building ./salsa20_core.ml:
#               - Failed to build all of these:
#                   - Building ./salsa20_core.mly
#                   - Building ./salsa20_core.mll
# pkg.ml: [ERROR] cmd ['ocamlbuild' '-use-ocamlfind' '-classic-display' '-j' '4' '-tag' 'debug'
#      '-build-dir' '_build' 'pkg.otarget']: exited with 6

[hannesm]

maybe a fix like #28959 works there as well