[hakari] [feature-request] Support non-workspace crates

[RalfJung]

I have a use for hakari (or something very much like it) in a non-workspace crate: some of my [dev-dependencies] cause features to be enable for crates that also appear (transitively) as regular [dependencies]. This means that even after I did cargo test, which builds the binary and test targets, if I now do cargo run that takes a minute because cargo re-builds the binary target with a different feature set for one of its dependencies. For a while I tried to manually add more [dependencies] to avoid this, but this regresses very easily (I never figured out a good way to have CI check this) and it's quite tedious to figure out what one has to add. So, would be great if hakari could automate that. :-)

However, right now when I try to enable hakari in my crate, that fails because there is no workspace:

Error: 
   0: for path /home/r/src/rust/miri/Cargo.toml, [workspace] section not found

Would it make sense to support hakari in a non-workspace setting as well? My plan was to add the "hack" crate as a path dependency to my main crate, which AFAIK should work fine even when there is no workspace.

EDIT: I have now found a different solution for Miri. But this could still be useful for someone else.