Questions to answer in further revisions:

Will installed packages be read-only (at least until deletion is called for), or strictly checksummed, or immutable by convention only? What consequences, if any, would modifications have? Using code that doesn't match what the manifest specifies was installed isn't good for reproducibility, but what's the intended level of tracking and granularity of this? If packages aren't always git repositories any more, how are generated files and downloaded resources, which would be ignored from a version control perspective, dealt with?

This ties into the bigger separate question of where and how development happens if everything Pkg3 touches is immutable (by convention only, or strictly enforced). If you want to make a local change, does that require a separate installation mechanism and modifiable copy that lives outside of Pkg3 somewhere? Or if you make it locally do you then have untracked local modifications that never get recorded anywhere? (People will forget they've made this kind of change if packages aren't git repos.) Most other package systems work this way, but most other package systems have an unfriendly distinction between the way users work with packages and the way developers/contributors do. The low barrier to entry of contributing to Julia packages is a huge benefit to our ability to get users to become developers.