This directory is meant for manually vendored dependencies, i.e. ones which are
either taken from tarballs or git repositories, rather than being vendored with
`cargo vendor` via the `scripts/maint/updateRustDependencies.sh` script.  The
dependencies in this directory must be present for builds, regardless of whether
--enable-cargo-online-mode was specified.  The reason for this is that there may
exist external dependencies which are not available through crates.io (and/or
via cargo), but which we wish to ship with by manually vendoring (similarly to
the rest of the C code in the `src/ext/` directory).

In order to explain how to add a dependency here, imagine you're working on the
following example crate `tor-foo` which lives at `src/rust/tor-foo`, and that
your `tor-foo` crate has a dependency on a crate, `bar`, which is not uploaded
to any registry.  To add `bar`, create a `src/ext/rust/vendor/bar-$COMMIT_ID`
directory here, where $COMMIT_ID is either some version number, commit id, or
globally unique identifier for the dependency version.  Let's say the `bar`
crate's $COMMIT_ID is "abcdef01".  Then, create a Cargo.toml.in at
`src/rust/tor-foo/Cargo.toml.in` and specify this dependency with:

[dependencies]
bar = { version = "=abcdef01", path = @srcdir@"/src/ext/rust/vendor/bar-abcdef01" }

(Yes, literally include the "=" before the $COMMIT_ID.)  Next, add
"src/rust/tor-foo/Cargo.toml.in" to the EXTRA_DIST list in
`src/rust/include.am`.
