unified-isos: Resolve symlinks

If the compose is configured to use symlinks for packages, the unified
ISO would include the symlinks which is useless.

Instead, let's check and replace any symlinks pointing outside of the
compose with the actual file.

JIRA: RHELCMP-13802
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
(cherry picked from commit 8ced384540)

pungi_utils/unified_isos.py

@@ -148,6 +148,13 @@ class UnifiedISO(object):
                     new_path = os.path.join(self.temp_dir, "trees", arch, old_relpath)
 
                 makedirs(os.path.dirname(new_path))
+                # Resolve symlinks to external files. Symlinks within the
+                # provided `dir` are kept.
+                if os.path.islink(old_path):
+                    real_path = os.readlink(old_path)
+                    abspath = os.path.normpath(os.path.join(os.path.dirname(old_path), real_path))
+                    if not abspath.startswith(dir):
+                        old_path = real_path
                 try:
                     self.linker.link(old_path, new_path)
                 except OSError as exc: