Support -packages flag

[cmoesel]

The IG Publisher supports a -packages flag for loading package tarballs from a directory:

-packages {packages directory}

Directory containing packages (tarball/gzips with implementation guide-associated artifacts with extension .tgz) to be used for packages to load without using package management infrastructure

Users have indicated on chat.fhir.org that it would be helpful for SUSHI to honor this feature as well.

In order for it to work when running the IG Publisher, I think the IG Publisher would also need to pass along this flag when it calls SUSHI.

Since these packages are not in the local FHIR cache, we would probably need to load them using FPL virtual packages.

Apparently all package tarballs in the folder need to be loaded, but the user can also specify these packages in their dependencies -- so we'd want to handle that cleanly.