New dependency management system

[obiwac]

I had initially described this in #50, but bringing that all in here:

Dependencies

Have some kind of list of Dep objects which looks something like this maybe:

deps = [
   Dep.local("/path/to/local/directory"),
   Dep.git("git://[email protected]/inobulles/iar", "v0.1.0"), # Could also be just 'main' e.g.
]

The process would be as follows:

• Copy or clone all the dependencies.
• Evaluate their dependencies in turn if they have a build.fl file (maybe through a command like bob list-deps).
• Once we have a list of all first-layer dependencies, we can copy/clone them and repeat this process until we've traversed the whole dependency tree.
• Build all these dependencies in parallel, layer-by-layer, starting from the leaf layer and working our way up. For Bob projects, figure out some way for them not to build their dependencies as we're already building them.
• Have some kind of fake install environment and install all the dependencies there.

Maybe it'd be cool to display the dependency tree while building, and for dependency lists to collapse once their parent has finished building.

For communicating changes in how many cores are available, look into eventfd?