[META] Deprecate the legacy include files

Drupal 8 has a bunch of the legacy include files.
As Drupal 9 planned to be released in 2020, it would be great to deprecate content of the legacy files as much as possible before release.

This is the issue for tracking the deprecation status of the legacy include files:

• bootstrap.inc
  • #2908079: Deprecate some of the bootstrap.inc PHP-related constants
  • #2347783: Deprecate drupal_get_path() and replace with ExtensionList::getPath()
• Current kernel includes
• common.inc
  • #3015538: [META] Deprecate contents of common.inc
• database.inc
  • #2319859: [META] Deprecate contents of database.inc
• module.inc
  • #2659940: Extension System, Part III: ThemeExtensionList and ThemeEngineExtensionList
  • #697946: Deprecate module_load_include() for removal before Drupal 9.0.x
• theme.inc
  • #2821399: [meta] Theme System Modernization Initiative
  • #3015812: Properly deprecate REGIONS_VISIBLE and REGIONS_ALL
• pager.inc
  • #2547833: Pager.inc -- add tests, clean it up, convert to a service, use it!
  • #2044435: Convert pager.inc to a service
• menu.inc
  • #2999961: [META] Deprecate contents of menu.inc
• tablesort.inc🎉🎉🎉@deprecated🎉🎉🎉
  • #3001201: Convert tablesort.inc functions to a static class
• file.inc
  • #2669074: Convert file_create_url() & file_url_transform_relative() to service, deprecate it
  • #2244513: Replace old managed and unmanaged file APIs with testable services (file.inc)
  • #2620304: htaccess functions should be a service
• unicode.inc🎉🎉🎉@deprecated🎉🎉🎉
• form.inc
  • #2959723: Create an interface and initial class for the batch processor service
• errors.inc
  • #3000229: Convert the error handler to a class
• schema.inc
  • #2124069: Convert schema.inc to a service
• entity.inc
  • #3000033: Deprecate entity_render_cache_clear function in entity.inc file
  • #2490966: [Meta] Replace deprecated usage of entity_create with a direct call to the entity type class
• Install includes
• install.core.inc
• install.inc
  • #2909480: Move REQUIREMENT_* constants out of install.inc file
• Rest of the files
• batch.inc
  • #2875151: Implement batch api as a service
• theme.maintenance.inc
• update.inc
  • #3012523: Convert the update.inc file functions to a class
• utility.inc
  • #3014752: Convert drupal_rebuild function into Cache static method