dzil: remove rendundant Repository plugin

The GithubMeta plugin already sets repo metadata correctly.
dzil: revise generation of no_index metadata

Changed to use the core Dist::Zilla plugin.  Removed the legacy 'ext'
directory and added 'xt'.
dzil: add comments to plugins without them
dzil: generate 'provides' metadata
dzil: move RunExtraTests later in dist.ini to avoid rebuilds during dzil test --all