register_cli_plugins() ran while mvt.ios and mvt.android were being
imported, so importing any part of MVT executed the entry points of every
installed command package. That made plugin loading depend on import
order: a plugin importing from MVT while MVT was still initializing got an
ImportError and was quietly demoted to a broken command, and the same
plugin worked when MVT happened to be imported first.
Move the call into a main() function in each CLI module and point the
console scripts at it, so registration happens once when the program
starts and importing MVT no longer runs third-party code. For packagers:
mvt.ios:cli and mvt.android:cli stay importable, but a wrapper invoking
cli() directly no longer registers the installed plugin commands and
should call main() instead.
