Works out how to turn GenServer modules into their virtual-time equivalents.

It is a planner, not a formatter: it reads a file, decides which tokens to replace and where, and hands the plan back. mix virtual_time.migrate decides whether to print that plan or apply it.

The plan is deliberately conservative — a call is only rewritten when the change provably means the same thing:

• use GenServer becomes use VirtualTimeGenServer, options untouched (use GenStateMachine → use VirtualTimeGenStateMachine);
• inside a converted module, Process.send_after/3, Process.sleep/1 and :timer.sleep/1 become VirtualTimeGenServer calls, because that is what puts the module on the virtual clock;
• GenServer.call/cast/stop/reply/start_link/start/child_spec is rewritten only when its target is a literal module that is converted somewhere in the project, or __MODULE__ inside a converted module.

Anything else — a pid, a variable, a module that stays a plain GenServer — is left alone, and reported when the caller is a converted module and the tool cannot tell. :erlang.send_after/3 is in that group: it takes the delay first where the virtual-time call takes it last, and swapping arguments is not a token replacement, so it is reported for a human to reorder.

Every edit carries the exact source text it expects to find at its position, and is checked against the source before it joins the plan, so a plan can never be applied to text it does not describe.