refactor(xdebug-handler): extract composer/xdebug_handler into the shirabe-xdebug-handler crate

Move `Composer\XdebugHandler` out of shirabe-external-packages and into
its own crate, so the path is `shirabe_xdebug_handler::XdebugHandler`
instead of
`shirabe_external_packages::composer::xdebug_handler::XdebugHandler`.

Standing alone, the stub no longer sits in a crate that shirabe-php-rpc
depends on, so drop the dependency-cycle rationale from the comments that
explain why callers reach for shirabe_php_rpc directly.

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

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