[Kernel] Add Kernel component

[alquerci]

Q	A
Branch?	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#29275, #27479
License	MIT
Doc PR	TODO

See #29275 for all details.

To help code review there are one commit per action.

Todo

• Release specifications, see questions on [RFC] [Kernel] Add the Kernel component #29275 (comment)
• Create the PR to add the kernel component (@alquerci)
  • List test cases
  • Implements tests cases (@alquerci)
  • Write how to migrate on UPGRADE-6.0.md (@alquerci)
• Create the PR for update the documentation

[alquerci]

This is temporary until the component is available on Packagist.

It will be updated to "symfony/kernel": "^4.3".

[alquerci]

The Travis failure is due to symfony/messenger outdated on Packagist bdf294f instead of 326b694.

[alquerci]

@Korbeil @DanieleGBX I heard that you want to help in order to speed-up the merge of the new Kernel component. Any help are welcome from anyone else wants to help.

This PR provides all required code modifications, but we still needs to upgrade the documentation and to write the UPGRADE_6.0.md file. And for sure code review are welcome.

You can see the full to do list on this PR description.

[alquerci]

As it left only to upgrade the documentation then removed [WIP].

cc @stof @nicolas-grekas @dunglas

[Simperfit]

what's the ETA of this ?

[alquerci]

@Simperfit As you can see the Dev is done except for the documentation PR that I am not able to do.

I am just waiting for the green fire in order to rebase on the latest master version (currently based on 682855f).

@nicolas-grekas @dunglas What is the ETA for the code review? What is the priority of this?

[fabpot]

I've just re-read this issue and some other related to the same. I don't think this is something we will ever do. Lately, we have decoupled things more. And Flex did part of the job. But, having big BC breaks is out of the equation with the maturity of the framework. We might do some small steps in this direction, but no big bang. I'm closing for these reasons.