externalsrc: Pass through npmsw URIs in SRC_URI

NPM shrinkwrap files need to stay in SRC_URI even when using
externalsrc so that npm_do_fetch can run to fetch the required
dependencies.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>