logging messages should go to stderr over stdout
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`