Fallback to building dependencies from crates.io when the path is not valid

[pedrocr]

Describe the problem you are trying to solve

When developing interconnected crates it's common to put them all in a single git repository and then use both path and version in Cargo.toml so that both running the local code and publishing to crates.io with version dependencies works well.

A similar setup is when you want to have each crate in its own git repository instead. The same setup works but requires the user to always have the local crates in the paths or else an error for missing Cargo.toml will be generated for any dependency that has both version and path.

Describe the solution you'd like

It would be good to just fallback to building the version from crates.io when the path doesn't exist or doesn't have a Cargo.toml inside it. That way someone that clones just the single repository will be able to do local changes, commits and PRs and only need to clone other crates if they want to do coordinated changes between crates.

[pedrocr]

For an example of this at work see the following crates:

https://github.com/pedrocr/chimper
https://github.com/pedrocr/imagepipe
https://github.com/pedrocr/multicache
https://github.com/pedrocr/rawloader

If you compile chimper straight from crates.io it will resolve all the dependencies and work fine, because all the crates specify both path and a version dependency. If however you clone the chimper repository locally you will now need to clone also the other three repositories or the build will fail. If all you wanted to change was something local to chimper it's inconvenient to have to clone all the other repositories just to have the dependencies.