cargo_embargo: Parse out raw_name from src_path

When cargo_embargo is configured to use `cargo metadata` instead of
running `cargo build`, we don't get the expected raw name for extern
crates unless we expect the source path. This in turn means we don't
generate the correct crate names for Trusty rules.mk files.

Test: manually, generate rules.mk for ciborium then build Trusty
Change-Id: Ie8c5b2868cd56b7f7e6d8294b41465682f5ac734
1 file changed