System packages enhancements

[siddhantk232]

I also want to simplify things. Replace system: <alias> with system: true, and instead of relying on alias, rely directly on package name. Top level package should be able to "overwrite" any module.

I am also happy to drop the system terminology, and make any package overridable by main package.

-- fastn.dependency: <original-package>
override: <the-module-that-will-be-passed-when-original-package-is-requested>

Originally posted by @amitu in #2139

These enhancements should definitely happen. Moving it to a newer issue so that I can close the original as done.