Break up db_engine_specs

[villebro]

CATEGORY

Choose one

• Refactor

SUMMARY

This PR changes db_engine_specs.py into a package with one module per engine, e.g. db_engine_specs/bigquery.py. All engines are automatically populated into db_engine_specs.engines like before (now in db_engine_specs/__init__.py), so only importing of specific engine specs will need to be changed:

Before: from superset.db_engine_specs import BaseEngineSpec
After: from superset.db_engine_specs.base import BaseEngineSpec

TEST PLAN

Local testing + CI

ADDITIONAL INFORMATION

• Has associated issue: Closes Break up db_engine_specs and add type hints #7412

REVIEWERS

@khtruong @john-bodley @betodealmeida @mistercrunch

[john-bodley]

Nice. I was thinking about making this same change. There's one small comment but overall LGTM.

[codecov-io]

Codecov Report

Merging #7676 into master will increase coverage by 0.12%.
The diff coverage is 69.25%.

@@            Coverage Diff             @@
##           master    #7676      +/-   ##
==========================================
+ Coverage   65.58%   65.71%   +0.12%     
==========================================
  Files         437      459      +22     
  Lines       21797    21873      +76     
  Branches     2403     2403              
==========================================
+ Hits        14295    14373      +78     
+ Misses       7381     7379       -2     
  Partials      121      121

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	81.98% <100%> (ø)	⬆️
superset/db_engine_specs/druid.py	100% <100%> (ø)
superset/db_engine_specs/__init__.py	100% <100%> (ø)
superset/db_engine_specs/gsheets.py	100% <100%> (ø)
superset/db_engine_specs/vertica.py	100% <100%> (ø)
superset/db_engine_specs/hive.py	43.75% <43.75%> (ø)
superset/db_engine_specs/drill.py	45% <45%> (ø)
superset/db_engine_specs/kylin.py	45.45% <45.45%> (ø)
superset/db_engine_specs/athena.py	46.66% <46.66%> (ø)
superset/db_engine_specs/impala.py	50% <50%> (ø)
... and 37 more

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[villebro]

Thanks for the quick comments @john-bodley, this should now be ready.

[mistercrunch]

LGTM - merge quick or conflict soon!

[villebro]

Pushing merge button in a few hours if no objections to avoid painful rebasing (already had 3 conflicts since initial commit).

[khtruong]

This is awesome. Thanks so much for this!