fix: Conditionally import required hook

This change makes the library to only import the hook it's going to need
based on the remote scheme.

As it is currently implemented, an environment that uses a S3 remote
fails if it doesn't have the `ssh` Airflow provider installer:

```
Traceback (most recent call last):
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/dbt.py", line 126, in get_remote
    return self.remotes[(scheme, conn_id)]
KeyError: ('s3', None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/dbt.py", line 300, in dbt_directory
    project_dir, profiles_dir = self.prepare_directory(
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/dbt.py", line 345, in prepare_directory
    project_dir_path = self.download_dbt_project(
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/dbt.py", line 158, in download_dbt_project
    remote = self.get_remote(scheme, self.project_conn_id)
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/dbt.py", line 128, in get_remote
    remote = get_remote(scheme, conn_id)
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/remote.py", line 147, in get_remote
    from .git import DbtGitRemoteHook
  File "/usr/local/airflow/.local/lib/python3.10/site-packages/airflow_dbt_python/hooks/git.py", line 5, in <module>
    from airflow.providers.ssh.hooks.ssh import SSHHook
ModuleNotFoundError: No module named 'airflow.providers.ssh'
```

The same happens when the environment uses a remote that requires the
SSH hook, and doesn't install the `aws` Airflow provider.

Author: adamantike

airflow_dbt_python/hooks/remote.py

@@ -144,15 +144,17 @@ def get_remote(scheme: str, conn_id: Optional[str] = None) -> DbtRemoteHook:
     In the future we should make our hooks discoverable and package ourselves as a
     proper Airflow providers package.
     """
-    from .git import DbtGitRemoteHook
-    from .localfs import DbtLocalFsRemoteHook
-    from .s3 import DbtS3RemoteHook
-
     if scheme == "s3":
+        from .s3 import DbtS3RemoteHook
+
         remote_cls: Type[DbtRemoteHook] = DbtS3RemoteHook
     elif scheme in ("https", "git", "git+ssh", "ssh", "http"):
+        from .git import DbtGitRemoteHook
+
         remote_cls = DbtGitRemoteHook
     elif scheme == "":
+        from .localfs import DbtLocalFsRemoteHook
+
         remote_cls = DbtLocalFsRemoteHook
     else:
         raise NotImplementedError(f"Backend {scheme} is not supported")