Proposal: Add IndexDomain for database of general indices

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• So far, we've stored general index entries to env.indexentries.
• This moves them all to new IndexDomain.
• adapter and collector for indexentries will be also replaced (in later step).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.0@cf50ec3). Click here to learn what that means.
The diff coverage is 86.66%.

@@          Coverage Diff           @@
##             2.0    #6543   +/-   ##
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  Coverage       ?   83.69%           
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  Files          ?      269           
  Lines          ?    40897           
  Branches       ?     5988           
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  Hits           ?    34227           
  Misses         ?     5344           
  Partials       ?     1326

Impacted Files	Coverage Δ
tests/test_build_html.py	97.61% <ø> (ø)
sphinx/application.py	78.29% <ø> (ø)
sphinx/environment/collectors/indexentries.py	0% <0%> (ø)
sphinx/environment/__init__.py	77.66% <100%> (ø)
sphinx/domains/index.py	94.11% <94.11%> (ø)

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[shimizukawa]

A domain is a collection of markup (reStructuredText directives and roles) to describe and link to objects belonging together, e.g. elements of a programming language.
http://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html

I thought "domain" is a set of directives and roles for each language, but "language" is just one use case.
Now I accept IndexDomain.

[shimizukawa]

LGTM

[tk0miya]

Merged. Thank you for reviewing!