test: Reproducer for refetching packages with file symlinks

This commit adds a test demonstrating the fact that even with the cache
packages which contain file symlinks are refetched on fresh builds. The
bug was discovered via a project that depends on the `menhir` project.

Signed-off-by: Puneeth Chaganti <punchagan@muse-amuse.in>

Author: punchagan

test/blackbox-tests/test-cases/pkg/fetch-cache-symlink.t

@@ -0,0 +1,69 @@
+Documenting that sources with file symlinks are re-fetched every time. The
+menhir package is an example of such a package in the opam-repository. See
+`./fetch-cache.t` for a normal package test.
+
+No need to set DUNE_CACHE (enabled by default) as the fetch rules are always
+considered safe to cache, but we'll set a custom directory for the shared
+cache.
+
+  $ export DUNE_CACHE_ROOT=$(pwd)/dune-cache
+  $ unset DUNE_CACHE
+
+Set up a project that depends on a package that is being downloaded. Note that
+the package being downloaded has a symlink.
+
+  $ make_lockdir
+  $ echo "Contents" > tar-contents
+  $ CONTENT_CHECKSUM=$(md5sum tar-contents | cut -f1 -d' ')
+  $ ln -s tar-contents tar-symlink
+  $ tar cf test.tar tar-contents tar-symlink
+  $ echo test.tar > fake-curls
+  $ SRC_PORT=1
+  $ SRC_CHECKSUM=$(md5sum test.tar | cut -f1 -d' ')
+  $ make_lockpkg test <<EOF
+  > (version 0.0.1)
+  > (source
+  >  (fetch
+  >   (url http://localhost:$SRC_PORT)
+  >   (checksum md5=$SRC_CHECKSUM)))
+  > EOF
+  $ cat > dune-project <<EOF
+  > (lang dune 3.17)
+  > (package (name my) (depends test) (allow_empty))
+  > EOF
+
+The first build should succeed, fetching the source, populating the cache and
+disabling the download of the source a second time.
+
+  $ build_pkg test
+
+The file that was fetched is in the cache. The symlink would have the same contents, so wouldn't appear separately in the cache.
+
+  $ find $DUNE_CACHE_ROOT/db/files -type f -exec md5sum {} \; | grep --quiet $CONTENT_CHECKSUM
+
+Cleaning the project to force rebuilding. If we attempt to build without the
+cache, it will fail, as the source is 404 now:
+
+  $ dune clean
+  $ export DUNE_CACHE=disabled
+  $ build_pkg test
+  File "dune.lock/test.pkg", line 4, characters 7-25:
+  4 |   (url http://localhost:1)
+             ^^^^^^^^^^^^^^^^^^
+  Error: Download failed with code 404
+         
+  [1]
+
+
+Enabling the cache again also attempts to re-download the source, since it contains a symlink:
+
+  $ dune clean
+  $ export DUNE_CACHE=enabled
+  $ build_pkg test
+  File "dune.lock/test.pkg", line 4, characters 7-25:
+  4 |   (url http://localhost:1)
+             ^^^^^^^^^^^^^^^^^^
+  Error: Download failed with code 404
+         
+  [1]
+