Use the packages/ directory rather than .meteor/meteorite/packages
Use system meteor unless specifying otherwise.
Still run meteor executable with PACKAGE_DIRS set.

This is so that you can still run `mrt test-packages X`
from within a package directory.
Write package symlinks to gitignore.
Fix bug where running mrt in a package would always update.
Bugfix
Fix bug where `mrt create` didn't work.
Don't create a packages/ dir if we don't need it.