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

[jfecher]

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

ModuleCache.relative_roots can be found here:

ante/src/cache/mod.rs

Line 47 in e38231f

 pub relative_roots: Vec<PathBuf>, 

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.