Remove registry_root from load_module

[y0z]

Motivation & Description of the changes

Since registry_root is "package" for most users, this PR removes the registry_root argument for simplicity.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.08%. Comparing base (e55a73d) to head (37c9a26).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   81.95%   82.08%   +0.13%     
==========================================
  Files           6        6              
  Lines         133      134       +1     
==========================================
+ Hits          109      110       +1     
  Misses         24       24              

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[toshihikoyanase]

Sounds reasonable to me.
If someone wants to change the registry_root, we can revisit this optional argument.