This was caused by -L calling setLogFormat() again, which caused the
creation of a new progress bar without destroying the old one. So we
had two progress bars clobbering each other.
We should change 'logger' to be a smart pointer, but I'll do that in a
future PR.
Fixes#6931.
Needed so that we can include it as a logger in loggers.cc without
adding a dependency on nix
This also requires moving names.hh to libutil to prevent a circular
dependency between libmain and libexpr
Make the printing of the build logs systematically go through the
logger, and replicate the behavior of `no-build-output` by having two
different loggers (one that prints the build logs and one that doesn't)
Add a new `--log-format` cli argument to change the format of the logs.
The possible values are
- raw (the default one for old-style commands)
- bar (the default one for new-style commands)
- bar-with-logs (equivalent to `--print-build-logs`)
- internal-json (the internal machine-readable json format)
