Discover subpackages and data.

        It also retrieves necessary files.

def find_packages(self, include):
        """
        Discover subpackages and data.

        It also retrieves necessary files.
        """
        pkgdir = None
        if include.source is not None:
            pkgdir = str(include.base)

        base = str(include.elements[0].parent)

        pkg_name = include.package
        pkg_data = defaultdict(list)
        # Undocumented distutils feature:
        # the empty string matches all package names
        pkg_data[""].append("*")
        packages = [pkg_name]
        subpkg_paths = set()

        def find_nearest_pkg(rel_path):
            parts = rel_path.split(os.sep)
            for i in reversed(range(1, len(parts))):
                ancestor = "/".join(parts[:i])
                if ancestor in subpkg_paths:
                    pkg = ".".join([pkg_name] + parts[:i])
                    return pkg, "/".join(parts[i:])

            # Relative to the top-level package
            return pkg_name, Path(rel_path).as_posix()

        excluded_files = self.find_excluded_files()
        for path, dirnames, filenames in os.walk(str(base), topdown=True):
            if os.path.basename(path) == "__pycache__":
                continue

            from_top_level = os.path.relpath(path, base)
            if from_top_level == ".":
                continue

            is_subpkg = "__init__.py" in filenames
            if is_subpkg:
                subpkg_paths.add(from_top_level)
                parts = from_top_level.split(os.sep)
                packages.append(".".join([pkg_name] + parts))
            else:
                pkg, from_nearest_pkg = find_nearest_pkg(from_top_level)

                data_elements = [
                    f.relative_to(self._path)
                    for f in Path(path).glob("*")
                    if not f.is_dir()
                ]

                data = [e for e in data_elements if not self.is_excluded(e)]
                if not data:
                    continue

                if len(data) == len(data_elements):
                    pkg_data[pkg].append(pjoin(from_nearest_pkg, "*"))
                else:
                    for d in data:
                        if d.is_dir():
                            continue

                        pkg_data[pkg] += [pjoin(from_nearest_pkg, d.name) for d in data]

        # Sort values in pkg_data
        pkg_data = {k: sorted(v) for (k, v) in pkg_data.items() if v}

        return pkgdir, sorted(packages), pkg_data