logging messages should go to stderr over stdout

[daylinmorgan]

Currently nimble writes all of it's logging messages to stdout but they should probably be going to stderr, particularly since
something like nimble dump --json with actual output data should be straightforward to pipe to another process or file.

If you run nimble dump --json in a project with a nimbledeps directory you'll also get Warning: Using project local deps mode making the output less usable.

Should be a simple change if all the logging is already routed through nimblepkg/cli.display

[Araq]

Fwiw I don't like stderr as it means >file.txt redirection doesn't work and you can never see the output stream that is used. The stderr/stdout distinction is more annoying than it is useful.