Name and order the probe code

Every package-level name of the probe starts with probe: its run
ProbeRun, the summarizer ProbeSlotSummarizer that limits the builds at
once, the reader of its log and of the usage file, and the parts of its
report. Names such as ratio, timeText, and modelLines read as helpers of
the whole program, but only the probe uses them.

The declarations of the probe files follow the order of the go-code
skill. The behavior does not change.

Issue: 947bddb