Fix extraction of package dir from package ID

After https://github.com/rust-lang/cargo/pull/12914, the format of the
package ID changed to be

    path+file:///path/crate_name#version

See https://doc.rust-lang.org/cargo/reference/pkgid-spec.html for
details on the format.

The test files were regenerated with `cargo metadata`. This procedure
is now documented in a `README.md` file for future generations.

Bug: 333808266
Test: atest --host cargo_embargo.test
Test: Ran `cargo_embargo generate cargo_embargo.json` on chrono
Change-Id: I055d19baf6e78f30b708296434082121762573af
15 files changed