[CQ] limit internal XDebugSessionImpl dependence; remove dead code

[pq]

🤫 Limits XDebugSessionImpl dependence (silencing two internal API access inspections); adding a note explaining our usage
🧹 Removes VmServiceWrapper dead code

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[jwren]

Change LGTM, but do test to validate that this code works back to the Dart 3.0 debugger (and associated Flutter SDK)