spk ls -v and debug messages

[jrray]

Recent changes to spk ls have added some (developer-oriented?) debug messages that appear any time spk ls -v is used. The -v flag on the ls command is in common use to see extra details about builds. Some separation between seeing this extra information and seeing debug messages is needed.

A similar situation exists for solver output, extra -v's are needed to see more details from the solver, but increasing verbosity also enables debug or trace messages that end users would not be interested in seeing.

One possible direction to go is to introduce a --debug flag as an alternative to making --verbose do double duty when configuring logging. Another option is to make more use of output channels in the tracing crate and keep developer-oriented out of the default channel and make use of RUST_LOG to enable those categories when needed. End users would generally never see these kinds of log messages.