Bugfix GCSToGCSOperator when copy files to folder without wildcard

[moiseenkov]

This PR fixes the following case.

The goal is to copy files with prefix source/foo.txt to the folder dest/ within a single GCS bucket.

1. Create a GCS bucket and upload two files to source directory like this:

gs://my-bucket/source/foo.txt
gs://my-bucket/source/foo.txt.abc
gs://my-bucket/source/foo.txt/subfolder/file.txt

2. Upload the following DAG to a Cloud Composer environment:

from airflow import DAG
from airflow.providers.google.cloud.transfers.gcs_to_gcs import GCSToGCSOperator
from datetime import datetime

with DAG(
    dag_id="gcs_to_gcs_fail_example",
    schedule_interval=None,
    catchup=False,
    start_date=datetime(2021,1,1)
) as dag:
    copy_file = GCSToGCSOperator(
        task_id="copy_file",
        source_bucket="my-bucket",
        source_object="source/foo.txt",
        destination_object="dest/",
    )
    copy_file

3. Run the DAG

Expected bucket state:

gs://my-bucket/source/foo.txt
gs://my-bucket/source/foo.txt.abc
gs://my-bucket/source/foo.txt/subfolder/file.txt
gs://my-bucket/dest/foo.txt
gs://my-bucket/dest/foo.txt.abc
gs://my-bucket/dest/foo.txt/subfolder/file.txt

Actual (incorrect) bucket state:

gs://my-bucket/source/foo.txt
gs://my-bucket/source/foo.txt.abc
gs://my-bucket/source/foo.txt/subfolder/file.txt
gs://my-bucket/dest/source/foo.txt
gs://my-bucket/dest/source/foo.txt.abc
gs://my-bucket/dest/source/foo.txt/subfolder/file.txt

[VladaZakharova]

Hi @potiuk !
Could we please review these changes?