Use `Cow::<'_, str>::deserialize` to ensure that it would still work
even if it contains escaped characters.
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>
Makes initializing `metafiles::CrateVersionSource` more readable and
improves performance since the parsing is now cached.
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>