When doing local development, users may wish to specify many `path`
dependencies that all live in the same local directory. If that local
directory is not a short distance from the `Cargo.toml`, this can get
unwieldy.

This patch introduces the notion of path "prefixes", which are defined
in `.cargo/config.toml`:

```toml
[path]
devdir = "/home/jon/dev/rust/"
```

Which can then be used as the root for path dependencies. For example:

```toml
foo = { path = "devdir::foo" }
bar = { path = "devdir::bar" }
baz = { path = "devdir::ws/baz" }
```

would fetch `foo` from `/home/jon/dev/rust/foo`, `bar` from
`/home/jon/dev/rust/bar`, and `baz` from `/home/jon/dev/rust/ws/baz`.

This feature also serves as a convenient way for external build systems
to mask build-dependent paths from the user. Consider a build system
that vendors first-party dependencies into

```
/home/user/workplace/feature-1/build/first-party-package/first-party-package-1.0/x86_64/dev/build/private/rust-vendored/
```

Instead of requiring users to hard-code that path, or the relative
equivalent, into their `Cargo.toml`, the build-system can instead
produce a project-local `.cargo/config.toml` that defines that path as
`path.vendored`, and the user can then use vendored dependencies using

```toml
foo = { path = "vendored::foo" }
```