externalsrc.bbclass: support crate fetcher on externalsrc

To support crate:// fetcher on externalsrc, we need to make pass-through
the URIs in SRC_URI.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>