Move hcloud.hcloud module to hcloud._client

[jooola]

Feature Request

Is your feature request related to a problem? Please describe.

The hcloud.hcloud module is set as if it can be used by users. We also reexport the Client class in hcloud/__init__.py. To have a clear definition of what the API exports and what API are usable by users, we should probably hide the hcloud.hcloud module.

Describe the solution you'd like

I propose to move the hcloud.hcloud module to hcloud._client. The Client class is already reexported in hcloud/__init__.py so most of the user should already use the "new" path.

In addition we can properly define all the exported APIs using the __all__ array.

Teachability, Documentation, Adoption, Migration Strategy

This is a breaking change, should be done for v2.0.0.