Persist is_syncing across container rebuilds

Pushed the changes I tested in #3105597: Stop copying block configuration from active theme when enabling a new theme into the MR.

Also updated the tested we added in the original issue to actually install a theme instead of just hardcoding the hooks. That then fails as expected on head.

I had to alter the test to use test_theme and not claro, because claro has its own config and then that also skips this logic.

I noticed that moduleInstaller already had its own version of this, I pushed that logic into DrupalKernel so it's shared between the two installers and works the same way.

@berdir explained the test changes to me in Slack. I think they make sense, and are backed up by a comment so we don't accidentally regress it in the future.

What's more, I like this solution. It fixes the regression in block_theme_initialize(), makes the extension system more consistent, and allows me to continue my armed truce with that (disliked) function...without needing to harbor a full-on vendetta.

Committed/pushed to main, 11.x and 11.3.x, thanks!

Hi, this may have caused a regression, see #3584145: Call to member function removeComponent error after updating to 11.3.6 or something else needs updating/fixing.
There are quite a few modules using \Drupal::isConfigSyncing() in their hook_install, see here.

Any thoughts? Thank you

Looks the following code path might have an issue:

• ModuleInstaller::install(), with the $module_list passed to it having count > 1
• This calls ModuleInstaller::doInstall(), then if the $module_list passed to doInstall() has count > 1, this calls ModuleInstaller::updateKernel([])
• Which calls $this->kernel->resetContainer(), and resetContainer does not persist the config installer sync status

Opened #3584812: ConfigInstaller isSyncing does not persist to hook_modules_installed() when multiple modules are installed to address #16.