intersphinx: Normalize mapping on config-inited

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• The main loop of load_mappings() is too complicated. To make it simple, this tries to normalize intersphinx_mapping on config-inited event.

[codecov]

Codecov Report

Merging #5826 into master will decrease coverage by 0.03%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master    #5826      +/-   ##
==========================================
- Coverage   83.29%   83.26%   -0.04%     
==========================================
  Files         290      292       +2     
  Lines       39290    39338      +48     
  Branches     5864     5868       +4     
==========================================
+ Hits        32728    32756      +28     
- Misses       5210     5227      +17     
- Partials     1352     1355       +3

Impacted Files	Coverage Δ
tests/test_ext_intersphinx.py	100% <100%> (ø)	⬆️
sphinx/ext/intersphinx.py	88.28% <84.21%> (-1.12%)	⬇️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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