Move formatters out of main package

[dmikusa]

Thoughts on this one?

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The primary purpose of this PR is to move the formatters that were only used for debug logging out of the main package.

Doing this has a few other side effects that are helpful as well:

• It moves the Logger interface out of the main package into Log. It didn't make a lot of sense that this interface was not in the log package. It was originally put there because that would not require libcnb to use the log package, but we are doing this anyway so we can default to the plain logger.
• It adds a formatter interface so one could customize the output of the directory contents. This is important if you are adding customization to the output format on top of libcnb, i.e. making your own logger.
• DirectoryContentsWriter is more generic now. It doesn't need customized for every directory to log.
• DirectoryContentsWriter now writes directly to a writer, where it previously build up a large string in memory. This could use an unbounded amount of memory given that we log contents of user controlled locations like the application directory.
• Errors are handled in a more sane way. They are still swallowed, but they bubble up to the appropriate location and are captured.

Signed-off-by: Daniel Mikusa dmikusa@vmware.com

[samj1912]

@dmikusa - I made some minor changes so that the default formatter (plain formatter) is internal only since it takes no args. I changed the interface for the content writer a bit to reuse the formatter/debug writer and had the write method take in both the title and the path.