Restructure transforms

[julia-shenshina]

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#403

[codecov-commenter]

Codecov Report

Merging #420 (3592368) into master (0e1f7c1) will increase coverage by 0.15%.
The diff coverage is 99.29%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   87.13%   87.28%   +0.15%     
==========================================
  Files          99      111      +12     
  Lines        5067     5127      +60     
==========================================
+ Hits         4415     4475      +60     
  Misses        652      652              

Impacted Files	Coverage Δ
...na/transforms/decomposition/change_points_trend.py	99.04% <ø> (ø)
etna/transforms/decomposition/detrend.py	98.33% <ø> (ø)
etna/transforms/decomposition/stl.py	94.02% <ø> (ø)
etna/transforms/encoders/segment_encoder.py	100.00% <ø> (ø)
etna/transforms/feature_selection/base.py	100.00% <ø> (ø)
etna/transforms/feature_selection/filter.py	100.00% <ø> (ø)
etna/transforms/math/add_constant.py	97.61% <ø> (ø)
etna/transforms/math/lags.py	100.00% <ø> (ø)
etna/transforms/math/log.py	97.82% <ø> (ø)
etna/transforms/math/sklearn.py	95.32% <ø> (ø)
... and 28 more

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[Mr-Geekman]

There are 40 lines in a new file and 41 in the old version. It is strange.

[julia-shenshina]

it's true
I didn't add import for BaseFeatureSelectionTransform at the top level of transforms because this class seems to be not very useful for users