Support local cargo dependencies outside of a workspace

[cameron-martin]

If you have a dependency on a local crate that isn't in the same workspace, cargo-bazel seems to fail with:

error: failed to load manifest for dependency `foo`

Caused by:
  failed to read `/foo/Cargo.toml`

My situation is almost identical to the multi-package example, but where there is a dependency between the packages, for example pkg_a has the following dependency:

pkg_b = { path = "../sub_pkgs/pkg_b" }

I know I could arrange these into a cargo workspace, but I would prefer not to if possible.

[bsilver8192]

This looks like the same problem as google/cargo-raze#361, for reference.

[cameron-martin]

Are you sure that is a duplicate? This is about rust to rust dependencies, not C++ to rust

[bsilver8192]

Oops, my bad, edited the prior comment to avoid further confusion. I was confused about which project this issue is in, I was trying to refer to google/cargo-raze#361 which is the same problem for raze.