The approach is to look in the parts directory and report everything
that has been installed in lib/ruby/gems/specifications/*.gemspec
To find the name and version, instead of trying to deduct from the
filename, we read the stub metadata from the gemspec file, using the
same login as what rubygems does in
https://github.com/rubygems/rubygems/blob/6b360fdcd2cbd72a902f4107a1deb264a6e7d3ca/lib/rubygems/stub_specification.rb#L4-L10