dhall package command: Added support for automatic sub-packages

[mmhat]

When you pass the --recursive flag (or -r for short) to the dhall package command, then for each directory passed as an argument a sub-package is created in each subdirectory, and that sub-package is included in the (main) package in addition to the *.dhall files of that directory.

For example, for a directory structure like

subdirectory1/nesteddirectory1/
subdirectory1/nesteddirectory2/
subdirectory2/

calling dhall package --recursive . is equivalent to:

# Create sub-package for subdirectory1/nesteddirectory1/ first
cd subdirectory1/nesteddirectory1
dhall package .
# Create sub-package for subdirectory1/nesteddirectory2/ next
cd ../nesteddirectory2
dhall package .
# Create sub-package for subdirectory1/
cd ..
dhall package . */package.dhall
# Create sub-package for subdirectory2/
cd ../subdirectory2
dhall package .
# Create main package
cd ..
dhall package . */package.dhall