Rationalize Logging/Tracing

[rgrinberg]

This feature is born out of working our private fork of dune. Experience has shown that tracing is quite important and our so called "logging" is otherwise redundant. I therefore propose the following way to unify them:

1. Enable event tracing by default
2. Remove the log file
3. Add all log events to the trace
4. Introduce a dune trace --log command to filter out the so called log messages to reproduce (at least morally) what is currently our log file

[Alizter]

So the trace files will be an enriched _build/log and will always be created? When we want to see the moral _build/log we would filter on this?

[pgiarrusso-sl]

#13015 did not fix this issue fully:

Fixes #12904 except for the last point. That will come in a subsequent PR.

Does such a PR exist?