plugin utils should be exported from the main module

[olsonpm]

In the documentation it is advised to reference an internal file

// The utility wrapper import path changes a bit when developing a custom plug-in.
const { wrap } = require('clay-log/plugins/_utils');

If you plan on having this plugin utility as part of the public api for clay-log, then it should instead be exported as part of the main module, e.g.

const { pluginUtils } = require('clay-log');

This clearly communicates that consumers can rely on this api without referencing files internal to clay-log. Internal files are traditionally considered unstable.