Initialize buildpack submodules

Previously, when a buildpack was cloned, any submodules containted
within that repository were not initialized.  This means that any
split of buildpack code across repositories was difficult if not
impossible.  This change adds --recurse-submodules to the clone
command to initialize any declared submodules.

[#50081011]
Merge pull request #23 from nebhale/50081011-submodules

Initialize buildpack submodules