* executable.dart#main() depends on runner.dart#run()
* Refactor code such that non-commands don't depend on commands.
No code was actually changed in this PR - code was merely moved from
point A to point B.
1. Add `PortScanner` abstraction so that we don't do actual port scanning
in tests.
2. Don't change the real `cwd` of the isolate during tests, as it affects
all tests, not just the current running test.
Fixes#8761
