Right now, the relevant steps of our release procedure are:

• create new branch
• in new branch, update crate version number and first-party dependencies from git to crates.io
• after release, cherrypick crate version change back into main

This is a bit of a hassle; I propose that instead we:

• in main, update crate version number and first-party dependencies
• create new branch

The only wrinkle is that we want main to always use the git version of first-party dependencies; happily, cargo supports specifying multiple locations for a dependency for exactly this use case, so we can just list both git and version for those and it'll work.