Add build flag to list and fetch dependencies

[rob05c]

I would like build system commands (e.g. an scons target) to:

1. list all dependencies
2. fetch and cache dependencies
3. build from fetched dependencies, without ever reaching out on the network.

This would let users package all the source necessary to build, so builds can be done without network, or if those URLs on the internet disappear.

It would also help with things like vulnerability checking, so users can easily see and search dependencies for CVEs, and update those dependencies.