Add tests

[ethowitz]

We should add tests for, at minimum, the following cases:

• A standalone crate
• A workspace with independent members
• A workspace with crates that share dependencies
• A workspace with crates that depend on each other
• Build scripts/OUT_DIR
• Proc macros
• A standalone crate nested under the root of a workspace from which it's excluded
• Multiple build targets in a single crate
• Feature flags

We'll want to validate the JSON output to ensure it has what we expect. We'll need to come up with a good way to write tests. Some strategies are:

• Include directories with the actual crate definitions (Cargo.toml & source files) under tests/ or something for each test case. Downside is that it will be annoying to write tests
• Add a type/macro to create workspaces/crates in tmp directories on the fly
• Some third thing that I haven't thought of yet