Dependencies referenced by path.

Example:
```
crate = { path = "../crate" }
```

This fails with
```
error: failed to load source for a dependency on `crate`

Caused by:
  Unable to update file:///crate

Caused by:
  failed to read `/crate/Cargo.toml`

Caused by:
  No such file or directory (os error 2)
```