refactor: replace the file storage logger with the default logger

[jinrenjie]

I use certmagic in my project, it's great, but I had trouble collecting logs, so I want to replace log with Default.Logger to make the log style uniform!

[mholt]

Thanks; I suppose this is better than using the std lib logger, as it's the "wrong" logger either way (these places don't have a config available to them for us to get the "correct" logger).

Although, Default is intended as a template and shouldn't be used directly. Plus, it could be modified to set Default.Logger to nil. ( 💥 ) We should probably refer to defaultLogger directly if possible. Would that work?

[jinrenjie]

@mholt I think Default.Logger is more suitable than defaultLogger, because users can set custom Logger through Default.Logger. In addition, I saw in the code that if Default.Logger is nil, defaultLogger can be assigned to Default.Logger.

[mholt]

In addition, I saw in the code that if Default.Logger is nil, defaultLogger can be assigned to Default.Logger

That's only the case if a new instance of a Config is made by using Default as an input (like a template).

defaultLogger is the only zap logger guaranteed to be properly set (non-nil).

[jinrenjie]

@mholt I have replaced Default.Logger with defaultLogger.