minor #66378 [FrameworkBundle] Build the configuration trees once for `reference.php` and `schema.json` (nicolas-grekas)

This PR was merged into the 8.2 branch.

Discussion
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[FrameworkBundle] Build the configuration trees once for `reference.php` and `schema.json`

| Q             | A
| ------------- | ---
| Branch?       | 8.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

`PhpConfigReferenceDumpPass` and `JsonSchemaConfigDumpPass` each built the configuration trees of all extensions on every dev container build. They now share them and drop them once both ran, so the peak memory of a build doesn't change, and both files stay byte-identical. On the Symfony Demo, the two passes take 5.7 ms instead of 9.7 ms, and a dev container build from scratch runs 2.1% fewer instructions (about 10 ms of 750).

Commits
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3a61dba3c87 [FrameworkBundle] Build the configuration trees once for `reference.php` and `schema.json`