[third-party][rust] Move third_party/rust-crates in-tree

Third-party Rust crates were previously vendored out of a separate
repository, third_party/rust-crates. This causes difficulties
due to the separate versioning of third_party/rust-crates from
fuchsia.git.

After this commit, third-party crates are vendored out of
third_party/rust_crates.

This commit also introduces a second BUILD.gn file for
rust-mirrors/rustyline in order to allow for a soft transition.
After this, rust-mirrors/rustyline will have its BUILD.gn file
removed and the transitional/ subdirectory will be deleted.

Test: compiled
Change-Id: I58570efff4fe91691f93ac90636b5d722521c316