Top-level command has unmet deps

[srak289]

• python3-sh
• python3-mako
• python3-lmdb
• python3-yaml
• python3-packaging

Are all required simply to run ./carthage/bin/carthage --help

[srak289]

I think it might be a better user experience for the install_dependencies command for carthage itself to run off of core python to make initial install / setup more friendly.