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

• [x] Rename swiftformat directory to swifttidy.
• [x] Create swiftformat directory that provides backward compatibility.
• [x] 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.