Introduce a global Cargo interpreter state that keeps track of enabled
features on each crate.

Before generating AST of a Cargo subproject, it downloads every
sub-subproject and resolves the set of features enabled on each of them
recursively. When it later generates AST for one its dependencies, its
set of features and dependencies is already determined.

The library name defaults to its package name, but it can be different.
For example:
- package name: cairo-sys-rs
- library name: cairo-sys
- dependency name: ffi

In the case the main project has a .wrap file for a cargo subproject,
that subproject's Cargo.lock must be loaded before we can recursively
fetch all its dependencies.

This returns --target value from rustc command line, or the native
target.