[feature] Immutable bit in package_config.json or package_graph.json

[dcharkes]

But we'd like to optimize caching behavior based on if something is immutable (e.g. published dependency, pub cache).

I'd rather provide that single bit (immutable or not), than exposing the kinds of source types.

Originally posted by @sigurdm in #4522

[sigurdm]

What definition of immutable should we use?

[sigurdm]

Is a package in the cache always immutable, or is it immutable when it and all of its dependencies transitively are cached?

[dcharkes]

Only itself, I'm using it to filter Dart sources, so if it's dependencies are an override those will not be in the hosted/ part of the pub-cache and those will be in the sources.

[dcharkes]

What definition of immutable should we use?

If the file path will never get different contents. (Basically we skip checking those files for changes. We only check the non-immutable Dart files, and the package_config.json. If you get a different hosted version of a package, it will be on a different file path, so the package_config.json changes.)