Cleaner imports

[marcpaterno]

Description

This refactoring restructures modules (typically single-file modules) into a well-organized multi-file structure.
This helps keep the file size manageable.
It also helps hide implementation details from module users, and clearly marks as private those items that are not intended for public use (use outside of the implementation).
It creates modules that have __all__ attributes which specify the public interface of the module.
It provides backwards compatibility with an important caveat: code that imports a name from a place other than the actual origin of that name will break.

Type of change

Please delete the bullet items below that do not apply to this pull request.

• Refactoring
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (08a6331) to head (324ec68).
⚠️ Report is 94 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          60       66    +6     
  Lines        5950     5963   +13     
  Branches      703      703           
=======================================
+ Hits         5950     5963   +13     

Files with missing lines	Coverage Δ
firecrown/fctools/sacc_convert.py	100.0% <ø> (ø)
firecrown/generators/inferred_galaxy_zdist.py	100.0% <100.0%> (ø)
firecrown/likelihood/binned_cluster.py	100.0% <100.0%> (ø)
...recrown/likelihood/binned_cluster_number_counts.py	100.0% <100.0%> (ø)
...elihood/binned_cluster_number_counts_deltasigma.py	100.0% <100.0%> (ø)
firecrown/likelihood/two_point.py	100.0% <ø> (ø)
firecrown/metadata_functions/__init__.py	100.0% <100.0%> (ø)
firecrown/metadata_functions/_combination_utils.py	100.0% <100.0%> (ø)
firecrown/metadata_functions/_extraction.py	100.0% <100.0%> (ø)
firecrown/metadata_functions/_matching.py	100.0% <100.0%> (ø)
... and 20 more

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