Add a command-line flag to configure the "relative_root" directories

Part of getting packages to work in Ante: https://antelang.org/docs/language/#packages

`ModuleCache.relative_roots` can be found here: https://github.com/jfecher/ante/blob/e38231ffa51b84a2ca53b4b0439d1ca5e0dea32a/src/cache/mod.rs#L47

We should have command-line flags for:
- Setting all of these directories at once, e.g. `--roots .:/path/to/stdlib:./deps`
- Adding a root without `--add-root /additional/path`
- Others?

The "relative root" name is not final. Feel free to rename it if you have a good suggestion, especially if it is a more familiar term from an existing language or program.

Read the package documentation and inspect ModuleCache.relative_roots in src/cache/mod.rs first. Trace the command-line argument entry point and determine how to support setting all roots and adding an individual root. Done means both flags configure package lookup correctly and the chosen terminology is reflected in the CLI help.

Written by the indexing model from the issue text.