The grab command downloads the dependencies of one or more Groovy
scripts to ./repository. This commit ensures that those previously
downloaded dependencies can be used by a subsequent invocation of the
run command.

The location and behaviour of the local cache is unaffected by this
change. If the dependencies in ./repository do not exist in the local
cache, Aether will "download" them from ./repository and store them in
the local cache.

Fixes #191