Unify the way to provide expression functions for the DI container

Function providers registered on the ContainerBuilder are now reused by
the PhpDumper when dumping the container instead of forcing to register
them again.
Closes #13012
bug #13013 Unify the way to provide expression functions for the DI container (stof)

This PR was merged into the 2.6 branch.

Discussion
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Unify the way to provide expression functions for the DI container

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #13012
| License       | MIT
| Doc PR        | n/a

Function providers registered on the ContainerBuilder are now reused by the PhpDumper when dumping the container instead of forcing to register them again.
I consider it as a bugfix, because the hook introduced in 2.6 is currently unusable.

Commits
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8425e4d Unify the way to provide expression functions for the DI container