`path` and `registry` shouldn't be mutually exclusive in dependency configurations

[sfackler]

In multi-crate workspaces, dependencies to other crates in the workspace are specified with path when building locally, as well as version when the crate is pulled from a registry. If registry can't be used with path, then multi-crate workspaces that publish to a custom registry don't work.

cc @withoutboats

[sfackler]

The check is made here:

cargo/src/cargo/util/toml/mod.rs

Line 930 in 868d373

(_, Some(_), Some(_)) => bail!("dependency ({}) specification is ambiguous. \

[sfackler]

Simply removing that restriction causes the registry field in the index entry to be the file path. It seems like the logic in #4568 is not quite right. The source ID of the dependency is going to be a file path here, which isn't right. cc @cswindle