opam external (system) deps

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Some opam deps need locally installed libs, e.g. libffi. This needs to be addressed for hermetic builds.

example, running `opam switch import switch.import --switch 4.10:

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
[ERROR] The compilation of ctypes failed at "/Users/gar/.opam/opam-init/hooks/sandbox.sh build make XEN=disable libffi.config".

#=== ERROR while compiling ctypes.0.18.0 ======================================#
# context     2.0.7 | macos/x86_64 | ocaml-base-compiler.4.10.2 | https://opam.ocaml.org#5c5a526e
# path        ~/.opam/4.10/.opam-switch/build/ctypes.0.18.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build make XEN=disable libffi.config
# exit-code   2
# env-file    ~/.opam/log/ctypes-64700-ad6566.env
# output-file ~/.opam/log/ctypes-64700-ad6566.out
### output ###
# [...]
# ./discover -ocamlc "ocamlfind  ocamlc" > libffi.config || (rm libffi.config && false)
# testing for brew: .............................. available
# testing for MacPorts: .......................... unavailable
# testing for pkg-config: ........................ available
# testing for libffi:Fatal error: exception Failure("You need to 'brew install libffi' to get a suitably up-to-date version")
# ocamlfind  ocamlc -o discover -package str -strict-sequence -linkpkg src/discover/commands.mli src/discover/commands.ml src/discover/discover.ml -I src/discover
# ./discover -ocamlc "ocamlfind  ocamlc" > libffi.config || (rm libffi.config && false)
# testing for brew: .............................. available
# testing for MacPorts: .......................... unavailable
# testing for pkg-config: ........................ available
# testing for libffi:Fatal error: exception Failure("You need to 'brew install libffi' to get a suitably up-to-date version")
# make: *** No rule to make target `libffi.config'.  Stop.