New SDK structure

[MarshalX]

not sure yet about it. new structure solves depending on the root package. for example on the old structure codegen depends on xrpc_client models... that codegen generates... (because of import aliases in the root package)

this PR breaks compatibility. update of imports are required

instead of from atproto.firehose import ... now we must use from atproto_firehose import ... (replace . to _)

some of aliases provided by atproto package:

__all__ = [
    'AsyncClient',
    'Client',
    'models',
    'CAR',
    'CID',
    'CIDType',
    'NSID',
    'AtUri',
    'AsyncFirehoseSubscribeLabelsClient',
    'AsyncFirehoseSubscribeReposClient',
    'FirehoseSubscribeLabelsClient',
    'FirehoseSubscribeReposClient',
    'parse_subscribe_labels_message',
    'parse_subscribe_repos_message',
    'firehose_models',
]

it means that imports like from atproto import Client will work fine without changes

new (completely different packages at the top of project):

old (one root package called "atproto" and many modules):