I decided to start things off fairly simple.  It either looks like the user is
specifying a manifest or they aren't.  This is assuming we only need to
handle `cargo foo.rs` and `cargo ./foo.rs` and not `cargo foo`.
I think in most shebang cases, path multiple components will present which makes
that a dead giveaway and likely to not overlap with aliases and
subcommands.

For reference, dlang's dub goes a lot further
1. Checks for the subcommand name being `-`
2. Checks if subcommand name ends with `.d`
3. Checks if subcommand name is built-in
4. Checks if a file with the subcommand name exists
5. Checks if a file with the subcommand name + `.d` exists

This would allow a `run.d` to override `dub-run` which doesn't seem
good.

This will give us a window to collect feedback on if this affects
anyone.

This is no where near the implementation we want but I think we should
develop incrementally on top of what we already have.

See https://github.com/epage/cargo-script-mvs/tree/main

This is written to reflect the current implementation though some parts
might read a little weird because I didn't want to write throw-away
documentation for when we change this.  For example, single-file
packages are currently only supported in `cargo <command>` and not as
manifest paths but this will change.