refactor(installed-versions): drop the Rust port, which has no readers

InstalledVersions is a runtime API for plugins and project code; Composer
itself never reads it. Its consumers run in the PHP worker against the copy
FilesystemRepository dumps to vendor/composer/InstalledVersions.php, whose
static state is already kept in sync by __shirabe_installed_versions_reload.
Nothing in Rust read the mirrored statics, so reload() and the reflection
setters were no-ops.

The tests covered only the Rust port, not the PHP class the worker loads, so
they assert nothing about compatibility; they are left as todo!() skeletons.
This also removes the shim functions method_exists, php_dir and
require_php_file, whose only caller was the deleted module.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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