buildcaches: fix directory link relocation (#26948)

When relocating a binary distribution, Spack only checks files to see
if they are a link that needs to be relocated. Directories can be
such links as well, however, and need to undergo the same checks
and potential relocation.

var/spack/repos/builtin.mock/packages/symly/package.py

@@ -0,0 +1,45 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+import os
+import sys
+
+from spack import *
+
+
+class Symly(Package):
+    """A toy package full of symlinks."""
+
+    homepage = "https://www.example.com"
+    has_code = False
+    version('3.0.0')
+
+    def install(self, spec, prefix):
+        symly_c = '''
+#include <stdio.h>
+
+int main() {
+    printf("I'm just here to give the build system something to do...");
+    return 0;
+}
+'''
+        mkdirp('%s/symly' % self.stage.source_path)
+        with open('%s/symly/symly.c' % self.stage.source_path, 'w') as f:
+            f.write(symly_c)
+        gcc = which('/usr/bin/gcc')
+        if sys.platform == 'darwin':
+            gcc = which('/usr/bin/clang')
+        mkdirp(prefix.bin)
+        mkdirp(prefix.lib64)
+        gcc('-o', 'symly.bin',
+            'symly/symly.c')
+        print("prefix.bin", prefix.bin)
+        copy('symly.bin', '%s/symly' % prefix.bin)
+        # create a symlinked file.
+        os.symlink('%s/symly' % prefix.bin,
+                   '%s/symly' % prefix.lib64)
+        # Create a symlinked directory.
+        os.symlink(prefix.bin, prefix.include)