Add symfony/flex dependency

[craigh]

This removes zikula/core dependency on symfony/symfony and instead requires needed components. Additionally, it adds dependency on symfony/flex. Flex is not useful in this state however except for core development (which seems minimal). Once this is merged, the distribution would need to be updated as well to ensure the flex dependency is used there. When this is done, real-world (downline) development starting with the distribution becomes possible using flex.

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	-
Refs tickets	#4352, #3601, #3438, #3962
License	MIT
Changelog updated	yes

Using flex has become increasingly needed with the premier of the symfony/ux initiative which relies on webpack-encore as well. I think we could implement webpack-encore without using it to "install" our primary front-end dependencies (bootstrap, jquery, etc). but instead use it initially only for the core.css file (and maybe the jQuery config file) and then, allow it to be used for downline development.

[Guite]

I really like this change.
But I vote for adding all Symfony components for 3.x to ensure BC.

Background: until #3438 is done the core can not ensure that no conflicts may occur because of extensions requiring additional Symfony components (in possibly too old or too new version). Hence, extensions need to rely on the core providing the entire Symfony ecosysystem. Of course this has limits (e.g. we should not include all possible notifier packages), but the most important components need to be kept available even if the core does not use them internally. One example for this is the messenger component.

[craigh]

@Guite, I have added the remaining components AFAIK except those noted in the updated changelog. Not noted there, these are also not added:

• symfony/inflector (deprecated)
• symfony/semaphore (experimental)
• symfony/uid (experimental)

I believe semaphore and uid were added in 5.2 anyway, so there is no reason to include them. inflector is to be replaced by the string component.

I've worked hard to ensure BC. I believe I've got all as needed. Please review again.

concerns addressed

[Guite]

Nice!

Adding uid soon will become useful though, but it is not urgent 😄

[craigh]

@Guite any further thoughts or concerns here? If not, I will merge this later when I have time. Looks like I will do it locally to deal with the conflicts.

[Guite]

No objections.