POSIX: Implement TryGetNormalizedPathImplementation and GetFinalPathName and properly handle firmlinks on Catalina

We need to implement TryGetNormalizedPathImplementation (or verify that paths across symlinks work properly with the current implementation).

TryGetNormalizedPathImplementation should return the canonical path of a file.

https://github.com/microsoft/VFSForGit/blob/69c3b45a253a710489222f954780c0a0c38d3bdc/GVFS/GVFS.Platform.POSIX/POSIXFileSystem.Shared.cs#L5

        public static bool TryGetNormalizedPathImplementation(string path, out string normalizedPath, out string errorMessage)
        {
            // TODO(POSIX): Properly determine normalized paths (e.g. across links)
            errorMessage = null;
            normalizedPath = path;
            return true;
        }

https://github.com/microsoft/VFSForGit/blob/69c3b45a253a710489222f954780c0a0c38d3bdc/GVFS/GVFS.NativeHooks.Common/common.posix.cpp#L14

PATH_STRING GetFinalPathName(const PATH_STRING& path)
{
    // TODO(Mac): Implement
    return path;
}

Update (7/30/2019)

See also the discussion here:

https://github.com/microsoft/VFSForGit/pull/1337#discussion_r307829313

We need to make sure that firmlinks are handled properly and that could require changes to GetRelativePath in PrjFSLib as well.