logging: Fix console logger instantiation

This commit fixes the console loggers so that messages are emitted
regardless of the debug log level. The problem was that in 3fcc875 we
updated the plugins to use a console logger obtained from the plugin
manager as opposed to a global logger instantiated in the plugins
package--the console logger obtained from the plugin manager was
instantiated in the runtime package by calling
logging.NewStandardLogger. Unfortunately, logging.NewStandardLogger
does not create a new logger--it returns the global logrus
logger.

This commit fixes the issue by deprecating logging.NewStandardLogger
and introducing two new functions in the logging package:

* logging.Get() - this replaces the old logging.NewStandardLogger
  function--this function should be called to obtain the debug logger
  used throughout OPA.

* logging.New() - this actually returns a new logger that can be
  configured independently from the debug logger used throughout
  OPA.

The runtime and sdk packages have been updated to call logging.New()
to obtain console loggers and the rest of the codebase has been
updated to call logging.Get() in place of logging.NewStandardLogger().

Fixes #3654

Signed-off-by: Torin Sandall <torinsandall@gmail.com>