Cleanup and refactoring of (rule|formatter)Loader

[ajafff]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

• deprecate getRelativePath, use path.resolve instead
• formatterLoader
  • allow uncompiled custom formatters (we already allow uncompiled rules)
  • try resolve formatter module relative to current directory first
  • add a test for loading a formatter package relative to cwd

Is there anything you'd like reviewers to focus on?

Using a formatter directly from node_modules without --formatters-dir is currently undocumented. I guess it makes sense to do it in this PR.

CHANGELOG.md entry:

tba