WIP Elevating skbio.stats.xxx to 1st-level modules

[qiyunzhu]

This PR elevates skbio.stats.xxx to 1st-level modules. Such that a user can do from skbio.composition import ancom. Whereas the original entries are kept for backward compatibility. This PR also standardizes the organization of skbio modules. Now, every module occupies a 1st-level subdirectory, with its own tests directory. This organization makes the codebase more modular. @wasade @mortonjt @mataton This is a proof-of-concept following our email discussion. I will follow up with a more detailed proposal of module organization.

Update: Per @mortonjt 's suggestion, I have reverted evolve, gradient, power and sampling back to stats, and kept only composition, distance, and ordination at the top level -- the three modules that deserve the most user attention.

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[codecov]

Codecov Report

Attention: Patch coverage is 98.77800% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 98.63%. Comparing base (4d14ad0) to head (6d7843a).

Files	Patch %	Lines
skbio/composition/_composition.py	98.49%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1992   +/-   ##
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  Coverage   98.63%   98.63%           
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  Files         179      180    +1     
  Lines       30678    30679    +1     
  Branches     7456     7456           
=======================================
+ Hits        30259    30260    +1     
  Misses        404      404           
  Partials       15       15           

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