add support for download mirrors

In Yocto builds, one may choose to use SOURCE_MIRROR_URL parameter in order to fetch sources from an external mirror (and only if sources are not found in the mirror, download them from the Internet).

The problem is that in such case source archives found in the external mirror are not downloaded in local DL_DIR so both tinfoilhat and aliensrc_creator cannot find them and return error.

This need to be changed, so that scripts look for sources first in SOURCE_MIRROR_URL and then in DL_DIR.