Lazily load constraint-solver instead of its mori

This should be a performance win (no need to load constraint-solver
unless you actually need to use it!), and it's what I wanted to do
initially instead of lazily loading mori, but it wasn't feasible with
the old super-recursive catalog.

This fixes an issue where running 'curmeteor rebuild' twice (!) died the
second time with a "Can't load npm module 'mori'" error. This is because
uniload (when run from a checkout) sets up Npm.require to read directly
from PACKAGE/.build.PACKAGE/npm/node_modules, which might get deleted
later in the process!  There are some complex and maybe slow ways to
resolve this general issue (copy the node_modules somewhere else?) but
for now, the easiest way to avoid the issue is just to load Npm modules
immediately inside packages which need to be uniloaded.
Comment about nodeModulesDirectories structure