Refactor data source classes to fix import issues

[achals]

Signed-off-by: Achal Shah achals@gmail.com

What this PR does / why we need it:

By keeping the Source classes in the same files as the offline store implementations, we can't import the sources without the corresponding libraries installed. This is a terrible experience. This PR moves the classes to a different file, so they can be imported for just the declarative use cases and not require boto or gcloud stuff to be installed.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #1723 (75b09d4) into master (8cfe914) will decrease coverage by 0.00%.
The diff coverage is 77.90%.

@@            Coverage Diff             @@
##           master    #1723      +/-   ##
==========================================
- Coverage   84.47%   84.46%   -0.01%     
==========================================
  Files          79       82       +3     
  Lines        7071     7093      +22     
==========================================
+ Hits         5973     5991      +18     
- Misses       1098     1102       +4     

Flag	Coverage Δ
integrationtests	84.39% <77.90%> (-0.01%)	⬇️
unittests	69.49% <58.42%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/feast/data_source.py	69.71% <45.45%> (-0.79%)	⬇️
...thon/feast/infra/offline_stores/redshift_source.py	67.81% <67.81%> (ø)
...thon/feast/infra/offline_stores/bigquery_source.py	80.68% <80.68%> (ø)
...k/python/feast/infra/offline_stores/file_source.py	88.73% <88.73%> (ø)
sdk/python/feast/__init__.py	90.00% <100.00%> (ø)
sdk/python/feast/infra/offline_stores/bigquery.py	81.10% <100.00%> (+0.50%)	⬆️
sdk/python/feast/infra/offline_stores/file.py	97.05% <100.00%> (+3.56%)	⬆️
sdk/python/feast/infra/offline_stores/redshift.py	91.66% <100.00%> (+14.74%)	⬆️
... and 1 more

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[achals]

/restest

[woop]

Why do we need __init__ in sdk?

[achals]

Why do we need __init__ in sdk?

we don't, intellij added it. removing

[woop]

This import is suspect. Should it be feast.bigquery_source?

[achals]

No, it's a relative import - should work fine (tests are happy too)

[woop]

I can't remember where this has caused problems for me, but I distinctly remember always having to make all imports absolute. Somewhere I ran into problems with relative imports. I don't think there are any relative imports in the code base, except the __init__.py files.

I'm fine leaving it as is and seeing if a problem pops up though.

[woop]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, woop

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