fileio calls mkdirs with basepath and not dirname(full_file_name)

When calling apache_beam.io.fileio.WriteToFiles with a file_naming argument that adds a directory to the path, the current implementation fails to write files if a mkdirs or analogous call is needed in the underlying file storage.

Example,

apache_beam.io.fileio.WriteToFiles(
    path="some/base/dir", sink=..., destination=lambda x: "events",
    file_naming=lambda *x: "subdir/file.txt"
) 

the current fileio implementation will call mkdirs with some/base/dir instead of some/base/dir/subdir.

The bug is currently at https://github.com/apache/beam/blob/67bcf1e16e3fdf68cdea7a4b42b9c003e4b8948c/sdks/python/apache_beam/io/fileio.py#L605.

====

Personally, I would recommend changing the FileSystems interface to have `open` call `mkdirs` in storages that require root parent directory creation.