[Proposal] Overhaul build system for better caching and customizability.

[d0cd]

💥 Proposal

snarkVM provides a Package which has a basic set of features, but is opinionated in how these objects can be constructed.
Leo allows for richer project structures and it's build system should not be restricted by the Package abstractions.
We propose updating Leo's system to borrow ideas from snarkvm::Package but is better fit for Leo's needs.