[DOP-5634] Move core classes to separate modules

[dolfinus]

Replace imports:

from onetl.core import DBReader
from onetl.core import DBWriter
from onetl.core import FileDownloader
from onetl.core import FileUploader
from onetl.core import FileMover
from onetl.core import FileFilter
from onetl.core import FileLimit

with:

# with fallback in onetl.core, showing deprecation warning
from onetl.db import DBReader
from onetl.db import DBWriter
from onetl.file import FileDownloader
from onetl.file import FileUploader

# no deprecation warning in onetl.core, this class was added only in 0.8.0
from onetl.file import FileMover

# legacy classes, left intact
from onetl.core import FileFilter  
from onetl.core import FileLimit

Also removed FileResult & FileSet from onetl.file module root, this is an implementation detail, not interface.

Documentation: https://onetl--46.org.readthedocs.build/en/46/

[codecov]

Codecov Report

Merging #46 (f91b655) into develop (890d088) will decrease coverage by 0.03%.
The diff coverage is 94.44%.

@@             Coverage Diff             @@
##           develop      #46      +/-   ##
===========================================
- Coverage    93.01%   92.98%   -0.03%     
===========================================
  Files          124      125       +1     
  Lines         5824     5832       +8     
  Branches      1008     1009       +1     
===========================================
+ Hits          5417     5423       +6     
- Misses         315      316       +1     
- Partials        92       93       +1     

Impacted Files	Coverage Δ
onetl/connection/db_connection/jdbc_connection.py	95.65% <ø> (ø)
onetl/connection/db_connection/jdbc_mixin.py	94.88% <ø> (ø)
onetl/connection/db_connection/mongodb.py	93.89% <ø> (ø)
onetl/core/file_filter/file_filter.py	95.18% <ø> (ø)
onetl/core/file_limit/file_limit.py	93.93% <ø> (ø)
onetl/db/db_writer/db_writer.py	84.44% <ø> (ø)
onetl/file/file_set.py	100.00% <ø> (ø)
onetl/hwm/store/memory_hwm_store.py	93.33% <ø> (ø)
onetl/hwm/store/yaml_hwm_store.py	100.00% <ø> (ø)
onetl/strategy/incremental_strategy.py	100.00% <ø> (ø)
... and 18 more