Group core components into broad categories

I took some specific notes while preparing my theme/render triage component list. This is only a very rough start and does not cover anything. It may also include components that are wrong or don't apply.

1. ajax system
   asset library system
   ...
   forms system
   ...
   render system
   ...
   theme system
   ...
   markup
   ...
   CSS
   Bartik theme
   Classy theme
   Seven theme
   Stark theme
   Stable theme
   

   Theme and render system

2. configuration entity system
   configuration system
   ...
   config.module
   

   Configuration system

3. database system
   mysql db driver
   postgresql db driver
   sqlite db driver
   database update system
   

   Database system

4. entity system
   ...
   field system
   file system
   ...
   image system
   ...
   typed data system
   ...
   comment.module
   ...
   datetime.module
   ...
   entity_reference.module
   field_ui.module
   file.module
   ...
   image.module
   ...
   link.module
   ...
   node system
   number.module
   ...
   options.module
   ...
   taxonomy.module
   telephone.module
   text.module
   

   Entity and Field system

5. language system
   ...
   transliteration system
   ...
   content_translation.module
   ...
   language.module
   ...
   locale.module
   ...
   translation.module
   translation_entity.module
   

   Translation and Localization system

6. menu system
   ...
   routing system
   ...
   menu.module
   menu_ui.module
   menu_link.module
   menu_link_content.module
   ...
   path.module
   ...
   

   Menu and routing system

7. request processing system
   ...
   xml-rpc system
   ...
   basic_auth.module
   ...
   hal.module
   ...
   jsonld.module
   ...
   rest.module
   ...
   serialization.module
   

   Request and Web Services system

8. block.module
   block_content.module
   ...
   layout.module
   

   Block and Layout system

9. help.module
   ...
   tour.module
   

   Help system

10. views.module
    views_ui.module
    

    Views system

#1 above could also be lumped into:

theme system
...
markup
...
CSS
Bartik theme
Classy theme
Seven theme
Stark theme
Stable theme

Theme system

and

ajax system
asset library system
...
forms system
...
render system

Render and Form system

Ideally in the future they would be better integrated but we are not there yet.

From @daffie in #2785891-38: The distinctions between modules, themes, and other subsystems are not relevant in MAINTAINERS.txt or the issue queue component field:

I know I am a little bit late, but I like the idea of working in teams and with this patch is the Database group still divided into the different databases. Can we combine them into one group and call that group something like "Database API (MySQL, PostgreSQL and SQLite)".

Interestingly, one thing @catch, @alexpott, @Cottser, @webchick, and I have discussed around this topic is whether it might be useful to change more of the file to the database pattern. So we might have something like:

Markup, CSS, and Themes
- Maintainer A
- Maintainer B

  Bartik
  - Maintainer C

  Classy
  - Maintainer C
  - Maintainer D

So that would list one team of maintainers A, B, C, and D, with their specific focus on the team (if any) listed as appropriate.

Also relevant:
#2786145: Clarify how multiple maintainers share a role

I think changing or combining any particular entries needs signoff from the maintainers from those subsystems, so we would want to find individual issues for each case.

Something I failed to document explicitly four years ago: This issue is postponed on #2449085: Make "Component" a multi-value field, or allow sub-components, because:

1. Changing the grouping/organization of core components requires the signoff of the respective subsystem maintainers
2. Basically every subsystem maintainer I spoke to about grouping their components said they would still want metadata about their sub-components.
3. Most subsystem maintainers had concerns about using tags to organize sub-components, because of the issues around discoverability, naming consistency, etc.