See instance method `path` instead of ivar `@path`

- To prepare for upcoming change, replace `path` with another method which generates
  proper tarball path
Package.new accepts pathes without ext or .json

- Package#initialize: Remove .tar.gz and .json extensions and set to
  @path_without_ext

- Package#path: Now generates .tar.gz path from @path_without_ext
- Package#meta_path: Now generates .json path from @path_without_ext