chore: refactor items to swifttidy package

[cgrindel]

Considerations

• Should we centralize configuration first? (i.e., avoid having to specify a reference to configuration for each invocation)

Tasks

• Rename swiftformat directory to swifttidy.
• Create swiftformat directory that provides backward compatibility.
• Only expose swiftformat_binary, swiftformat_library, and swiftformat_test via swiftformat package. Deprecating these helpers.

[cgrindel]

Opting not to centralize configuration because the Gazelle plugin will do the heavy lifting.