Build the tree for this package.

def build_tree(self):
        """Build the tree for this package."""
        for m in listdir(self.path):
            abs_m = join(self.path, m)
            if isfile(abs_m) and m.endswith('.py'):
                name = splitext(m)[0]
                if not self.limit_to or name in self.limit_to:
                    self.modules.append(Module(name, abs_m, self.dsm, self))
            elif isdir(abs_m):
                if isfile(join(abs_m, '__init__.py')) or not self.enforce_init:
                    heads, new_limit_to = self.split_limits_heads()
                    if not heads or m in heads:
                        self.packages.append(
                            Package(m, abs_m, self.dsm, self, new_limit_to,
                                    build_tree=True,
                                    build_dependencies=False,
                                    enforce_init=self.enforce_init))