Detect case-insensitive filesystems and prevent double-linking

This came out here: https://discuss.ocaml.org/t/dune-links-base-cmxa-twice/3465

I am not sure how hard or reasonable is to integrate this as a fix in dune, but I agree with @avsm that it is worth raising it here to discuss the eventuality.

I report the failure from the discuss thread below:

$ cat dune
(executable
  (name hcl)
  (libraries Base Stdio Core_kernel cryptokit cmdliner sexplib)
  (preprocess (pps ppx_jane))
  (modules_without_implementation asttypes cctypes))

$ dune build
[...]
(cd _build/default && /Users/leandros/.opam/4.07.1/bin/ocamlopt.opt -w @a-4-29-40-41-42-44-45-48-58-59-60-40 -strict-sequence -strict-formats -short-paths -keep-locs -warn-error -A -g -o hcl.exe -I /Users/leandros/.opam/4.07.1/lib/Base -I /Users/leandros/.opam/4.07.1/lib/Core_kernel -I /Users/leandros/.opam/4.07.1/lib/Stdio -I /Users/leandros/.opam/4.07.1/lib/base -I /Users/leandros/.opam/4.07.1/lib/base/caml -I /Users/leandros/.opam/4.07.1/lib/base/md5 -I /Users/leandros/.opam/4.07.1/lib/base/shadow_stdlib -I /Users/leandros/.opam/4.07.1/lib/base_bigstring -I /Users/leandros/.opam/4.07.1/lib/base_quickcheck -I /Users/leandros/.opam/4.07.1/lib/bin_prot -I /Users/leandros/.opam/4.07.1/lib/bin_prot/shape -I /Users/leandros/.opam/4.07.1/lib/cmdliner -I /Users/leandros/.opam/4.07.1/lib/core_kernel/base_for_tests -I /Users/leandros/.opam/4.07.1/lib/cryptokit -I /Users/leandros/.opam/4.07.1/lib/fieldslib -I /Users/leandros/.opam/4.07.1/lib/jane-street-headers -I /Users/leandros/.opam/4.07.1/lib/parsexp -I /Users/leandros/.opam/4.07.1/lib/ppx_assert/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_bench/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_compare/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_enumerate/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_expect/collector -I /Users/leandros/.opam/4.07.1/lib/ppx_expect/common -I /Users/leandros/.opam/4.07.1/lib/ppx_expect/config -I /Users/leandros/.opam/4.07.1/lib/ppx_hash/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_inline_test/config -I /Users/leandros/.opam/4.07.1/lib/ppx_inline_test/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/ppx_module_timer/runtime -I /Users/leandros/.opam/4.07.1/lib/ppx_sexp_conv/runtime-lib -I /Users/leandros/.opam/4.07.1/lib/sexplib -I /Users/leandros/.opam/4.07.1/lib/sexplib0 -I /Users/leandros/.opam/4.07.1/lib/splittable_random -I /Users/leandros/.opam/4.07.1/lib/stdio -I /Users/leandros/.opam/4.07.1/lib/time_now -I /Users/leandros/.opam/4.07.1/lib/typerep -I /Users/leandros/.opam/4.07.1/lib/variantslib -I /Users/leandros/.opam/4.07.1/lib/zarith /Users/leandros/.opam/4.07.1/lib/base/caml/caml.cmxa /Users/leandros/.opam/4.07.1/lib/base/shadow_stdlib/shadow_stdlib.cmxa /Users/leandros/.opam/4.07.1/lib/sexplib0/sexplib0.cmxa /Users/leandros/.opam/4.07.1/lib/base/base.cmxa /Users/leandros/.opam/4.07.1/lib/variantslib/variantslib.cmxa /Users/leandros/.opam/4.07.1/lib/typerep/typerep_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_sexp_conv/runtime-lib/ppx_sexp_conv_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_compare/runtime-lib/ppx_compare_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_enumerate/runtime-lib/ppx_enumerate_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_hash/runtime-lib/ppx_hash_lib.cmxa /Users/leandros/.opam/4.07.1/lib/stdio/stdio.cmxa /Users/leandros/.opam/4.07.1/lib/jane-street-headers/jane_street_headers.cmxa /Users/leandros/.opam/4.07.1/lib/time_now/time_now.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_module_timer/runtime/ppx_module_timer_runtime.cmxa /Users/leandros/.opam/4.07.1/lib/fieldslib/fieldslib.cmxa /Users/leandros/.opam/4.07.1/lib/base/md5/md5_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ocaml/unix.cmxa /Users/leandros/.opam/4.07.1/lib/ocaml/bigarray.cmxa /Users/leandros/.opam/4.07.1/lib/bin_prot/shape/bin_shape_lib.cmxa /Users/leandros/.opam/4.07.1/lib/bin_prot/bin_prot.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_bench/runtime-lib/ppx_bench_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_assert/runtime-lib/ppx_assert_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_expect/common/expect_test_common.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_expect/config/expect_test_config.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_inline_test/config/inline_test_config.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_inline_test/runtime-lib/ppx_inline_test_lib.cmxa /Users/leandros/.opam/4.07.1/lib/ppx_expect/collector/expect_test_collector.cmxa /Users/leandros/.opam/4.07.1/lib/splittable_random/splittable_random.cmxa /Users/leandros/.opam/4.07.1/lib/base_quickcheck/base_quickcheck.cmxa /Users/leandros/.opam/4.07.1/lib/Base/base.cmxa /Users/leandros/.opam/4.07.1/lib/Stdio/stdio.cmxa /Users/leandros/.opam/4.07.1/lib/base_bigstring/base_bigstring.cmxa /Users/leandros/.opam/4.07.1/lib/core_kernel/base_for_tests/base_for_tests.cmxa /Users/leandros/.opam/4.07.1/lib/parsexp/parsexp.cmxa /Users/leandros/.opam/4.07.1/lib/sexplib/sexplib.cmxa /Users/leandros/.opam/4.07.1/lib/Core_kernel/core_kernel.cmxa /Users/leandros/.opam/4.07.1/lib/zarith/zarith.cmxa /Users/leandros/.opam/4.07.1/lib/cryptokit/cryptokit.cmxa /Users/leandros/.opam/4.07.1/lib/cmdliner/cmdliner.cmxa .hcl.eobjs/ast.cmx .hcl.eobjs/parser.cmx .hcl.eobjs/lexer.cmx .hcl.eobjs/cc.cmx .hcl.eobjs/hcl.cmx)
File "_none_", line 1:
Error: Files /Users/leandros/.opam/4.07.1/lib/Base/base.cmxa
       and /Users/leandros/.opam/4.07.1/lib/base/base.cmxa
       both define a module named Base__Poly0

Start by reproducing the reported dune build on a case-insensitive filesystem using the shown dune file and compiler command. Trace how dune handles filesystem names and linking when both Base and base paths are present. Done means detecting the filesystem behavior and preventing the duplicate link that produces the Base__Poly0 error.