Rename 'report'

[khaeru]

• Use preferred module names:
  • message_ix.reporting → message_ix.report
  • reporting.computations → report.operator
  • Install an ixmp.util.DeprecatedPathFinder that supports imports from previous locations and raises DeprecationWarning.
  • Adjust imports within message_ix, tutorials, and references in docs.
• Improve documentation:
  • Address Sphinx nitpicks.
  • Add .. automodule:: message_ix for the top-level module; this allows other projects to link to :mod:'message_ix' directly through intersphinx.
• Improve the MESSAGE, MACRO, and MESSAGE_MACRO model classes, as enabled by Rename 'report', 'util' + miscellaneous clean-ups ixmp#500:
  • Include all items (including var and equ) in explicit lists of model items.
  • Add descriptions for items.
  • Derive reporting settings (RENAME_DIMS) from .models.
• Simplify report/__init__.py.

How to review

• Read the diff.
• Note the CI checks all pass.
• View the documentation, including what's new/migration notes, and ensure the changes are described clearly.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #761 (b988abf) into main (f360dc7) will increase coverage by 0.4%.
The diff coverage is 99.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #761     +/-   ##
=======================================
+ Coverage   94.3%   94.8%   +0.4%     
=======================================
  Files         43      43             
  Lines       3474    3797    +323     
=======================================
+ Hits        3279    3600    +321     
- Misses       195     197      +2     

Files	Coverage Δ
message_ix/__init__.py	91.6% <100.0%> (+1.1%)	⬆️
message_ix/core.py	95.1% <100.0%> (ø)
message_ix/macro.py	96.7% <100.0%> (+<0.1%)	⬆️
message_ix/report/__init__.py	100.0% <100.0%> (ø)
message_ix/report/operator.py	97.1% <100.0%> (ø)
message_ix/report/pyam.py	100.0% <ø> (ø)
message_ix/tests/report/test_operator.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_storage.py	100.0% <ø> (ø)
message_ix/tests/test_macro.py	100.0% <100.0%> (ø)
message_ix/tests/test_models.py	100.0% <100.0%> (ø)
... and 8 more