* Seems like Nix can get unhappy if a path fragment is evaluated too
eagerly, giving errors like
`error: access to absolute path '/Cargo.toml' is forbidden in pure eval mode (use '--impure' to override)`
* Changing to using string manipulation seems to resolve the issue
* Rather than recursively look up all paths in a given project and then
filtering for cargo-specific files, we now use our own customized
traversal method to only capture the results we want.
* There are some edge cases where cargo will allow a lib.rs file to be
present but not a main.rs (e.g. a binary cannot be called "examples",
which will happen if the crate is called that and a main.rs file is
present)
