Workspace configuration of flux crate confuses rustc

[rockstar]

Preface: I need to unpack some things I've noticed about the workspace configuration of libflux. A lot of this is just guessing, but I'm starting to see a pattern.

Right now, libflux is a cargo workspace, meaning it's a collection of crates. Inside of that workspace, we have the flux crate. Through just the evolutionary process, that crate source is found at libflux/src/flux, whereas a normal crate-in-workspace would be in libflux/flux/src. There are some configurations in the workspace to support this non-standard sort of approach, so in general, it should be fine.

Unfortunately, it seems that it confuses rustc's ability to figure out what needs to be compiled. For instance, if I cargo clippy, change a file, and cargo clippy again, rustc signals that there are no changes. What's odd is that with cargo build, change a file, and cargo build again, rustc determines that everything needs to be rebuilt[1].

It's possible (and in the case of cargo build, highly likely) that there are other moving parts at play, but this is not an issue I expect to see in a workspace using default configuration. I suggest making the flux crate more idiomatic, swapping where the src folder lives. At that point, we eliminate a non-common codepath, which makes it easier to track down these issues.

[1] One of the other potential issues with build times is related to external build tools and the complexities of build.rs. That's another moving piece that may need to be addressed.