Allow to create sublibraries named "opam"

Expected Behavior

Be able to have a sublibrary named opam (e.g. <pkg>.opam)

Actual Behavior & Reproduction

$ cat dune
(library
 (name test)
 (public_name test.opam))
$ touch test.ml test.opam
$ dune build
Error: symlink: _build/install/default/lib/test/opam/test.ml: Not a directory
Error: symlink: _build/install/default/lib/test/opam/test.cmi: Not a
directory
Error: symlink: _build/install/default/lib/test/opam/test.cmt: Not a
directory
Error: symlink: _build/install/default/lib/test/opam/test.cma: Not a
directory
Error: symlink: _build/install/default/lib/test/opam/test.cmx: Not a
directory
Error: symlink: _build/install/default/lib/test/opam/test.a: Not a directory
Error: symlink: _build/install/default/lib/test/opam/test.cmxa: Not a
directory
Error: symlink: _build/install/default/lib/test/opam/test.cmxs: Not a
directory

Specifications

• Version of dune (output of dune --version): 2.7.1

Start with the reported dune file and reproduce the failure using a public library named test.opam and the listed test.ml and test.opam files. Trace the installation or symlink handling that treats opam as a file rather than a directory; done means dune build succeeds and the sublibrary is installable under the expected path.