Replace `wheel2json` with a `wheel-inspect` command with `show`, `verify`, & `schema` subcommands

[jwodder]

• show (or info?) performs the actual inspection
• verify: Add a CLI command for just validating a wheel and only outputting errors #18
• schema dumps the schema used by show's output (Give the CLI an option for outputting the output JSON Schema #8)
• Alternatively, just have a wheel-inspect command with no subcommands, with the "validate" and "schema" behaviors invoked via --validate and --dump-schema options