fetch2/local: Don't default to files in DL_DIR for file:// urls

fetch2/local: Don't default to files in DL_DIR for file:// urls

Defaulting to any file in DL_DIR as the first match for a file:// url
doesn't make much sense and can lead to unexpected results.

This patch changes the logic so this is the last fallback location
instead. Whether it should be using DL_DIR at all for this is a
good question but something for another patch.

[YOCTO #1710]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>