Otherwise we'll need to duplicate the "get groups, then append
plugins" code when we add command information to datalad.api.__doc__.

This is cleaner because it avoids building up two lists and because
downstream code that works with the description doesn't have to know
the order, just the group name.

We'll use the list of command summaries in datalad.api.__doc__.

This will be used outside of the command-line interface.

An upcoming commit will use this to add command summaries in
datalad.api.__doc__.

An upcoming commit will use this to add command summaries in
datalad.api.__doc__.

Make it easier for users to discover what methods are available by
looking at the help for the api module.  One concern with this change
could be that we're bringing in more modules than before, but
inspecting sys.modules suggests that this is not the case.

There is still some code duplication between api._command_summary and
main.setup_parser that could pulled out, but that can be done later if
someone thinks it's worth the effort.

Closes datalad#2718.

Re: datalad#2718

Re: datalad#2728 (comment)