rename index.js to cli.js, readpkg to getenv
try to load sub-command from module.paths/ $NODE_PATH, before local mojito

note that this means that suers who want to install sub-commands globally
will also need to set $NODE_PATH to contain `npm root -g`. The natural set
of paths node looks in otherwise doesn't include the place where npm libs
go with `npm i -g` (as distinct from executables in $PATH).

    % node -pe module.paths
    [ '/Users/isao/Repos/wip/clistuff/node_modules',
      '/Users/isao/Repos/wip/node_modules',
      '/Users/isao/Repos/node_modules',
      '/Users/isao/node_modules',
      '/Users/node_modules',
      '/node_modules' ]
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