source loader class doesn't use logger due to early Loggable include

because [SourceLoader is used before logging is set up](https://github.com/elastic/logstash/blob/master/logstash-core/lib/logstash/runner.rb#L194-L247), this class will not respect the logging setup.

This was the initial reason for the PR https://github.com/elastic/logstash/pull/7970, you can see the different logging format in this example:

```
% bin/logstash -f "logstash_configs/*"
[2017-08-10T12:48:01,463][WARN ][logstash.config.source.multilocal] Ignoring the 'pipelines.yml' file because modules or command line options are specified
[2017-08-10T12:48:02,760][INFO ][logstash.config.source.local.configpathloader] No config files found in path {:path=>"/Users/joaoduarte/projects/elastic/logstash/logstash_configs/*"}
[ERROR] 2017-08-10 12:48:02.781 [Ruby-0-Thread-1: /Users/joaoduarte/projects/elastic/logstash/lib/bootstrap/environment.rb:6] sourceloader - No configuration found in the configured sources.
```