fix: ensure real path for header_mappings_dir

[wjaykim]

If header_mappings_dir is set to symlinked path, the header directory structure is not preserved currently because cocoapods uses header's real path to find relative path from header_mappings_dir, but header_mappings_dir is not ensured to be a real path(it could be symlinked path).

We can easily reproduce issue with following conditions:

• React Native 0.73+: it uses header_mappings_dir in its local podspecs
• with use_frameworks! :linkage => :static.
• pnpm: React Native add podspecs from Node.js' node_module directory, and pnpm uses symlink to create node_module directory.

Related issue:

• 0.73 fails to build on iOS with use_frameworks! :static due to yoga header path issue facebook/react-native#41938
• Regression -- 0.72 fails to compile on iOS with use_frameworks! :linkage => :static when using pnpm facebook/react-native#38140