model reorg: flatten model.preemption

restructuring/analysis/basic_facts/preemption/job/limited.v

...uring/analysis/basic_facts/preemption/job/nonpreemptive.v

...ucturing/analysis/basic_facts/preemption/job/preemptive.v

.../analysis/basic_facts/preemption/rtc_threshold/floating.v

...g/analysis/basic_facts/preemption/rtc_threshold/limited.v

...ysis/basic_facts/preemption/rtc_threshold/nonpreemptive.v

...nalysis/basic_facts/preemption/rtc_threshold/preemptive.v

...ructuring/analysis/basic_facts/preemption/task/floating.v

restructuring/analysis/basic_facts/preemption/task/limited.v

...ring/analysis/basic_facts/preemption/task/nonpreemptive.v

...cturing/analysis/basic_facts/preemption/task/preemptive.v

restructuring/model/preemption/fully_nonpreemptive.v

restructuring/model/preemption/fully_preemptive.v

restructuring/model/preemption/limited_preemptive.v

restructuring/model/task/preemption/floating_nonpreemptive.v

restructuring/model/task/preemption/limited_preemptive.v