Don't watch all sources of other packages in the same workspace, but only Cargo.* files

Right now, when you launch bacon on a package in a workspace, all the packages of that workspace are watched, even if they're not dependencies of the current package.

But it's probably enough to watch the Cargo.toml and Cargo.lock at the workspace level and in other packages of the workspace. Source/Test files in other packages which aren't dependencies can't impact the package.

Any different insight on that ?
([related survey](https://miaou.dystroy.org/4683?bacon#17693001))

Start at the workspace/package file-watching entry point and trace how Cargo.toml and Cargo.lock are selected for workspace packages versus source and test files. Verify the behavior with a workspace containing unrelated packages; done means changes in unrelated package sources no longer trigger bacon, while relevant Cargo.* changes remain watched.

Written by the indexing model from the issue text.