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In #20619 we overhauled how we were gathering stats for Windows processes. Unlike in Linux where we can ask for processes in a cgroup, on Windows we have to make a single expensive syscall to get all the processes and then build the tree ourselves. Our algorithm to do so is recursive and quadratic in both steps and space with the number of processes on the host. For busy hosts this hits the stack limit and panics the Nomad client. We already build a map of parent PID to PID, so modify this to be a map of parent PID to slice of children and then traverse that tree only from the root we care about (the executor PID). This moves the allocations to the heap but makes the stats gathering linear in steps and space required. This changeset also moves as much of this code as possible into an area not conditionally-compiled by OS, as the tagged test file was not being run in CI. Fixes: https://github.com/hashicorp/nomad/issues/23984