Name and order the compactor code

Every package-level name that only the compactor uses starts with
compactor: its limit of tries, the ruler, the task text, the note on a
long line, and the helpers that cut a line and pick the shortest one.
Names such as ruler and shortest read as helpers of the whole program,
but only the compactor uses them.

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

Issue: 947bddb