package/pkg-generic.mk: use site method for same-site extra downloads

When a package specifies extra downloads, it has the option to only name
the basename of the extra download, in which case that extra download
will be retrieved from the same location the main download is retrieved
from.

In that case, if the extra download contains a '+', it would confuse the
dl-wrapper, which believes the LHS of the '+' is the site method, and
the RHS the actual URI, and so the dl-wrapper mangles and damages the
URI when fetching such extra downloads, like that happens with android
tools, where the proper URI and mangled URIs of the extra download are,
respectively:

    https://launchpad.net/ubuntu/+archive/primary/+files/android-tools_4.2.2+git20130218-3ubuntu41.debian.tar.gz
    http://archive/primary/+files/android-tools_4.2.2+git20130218-3ubuntu41.debian.tar.gz



We fix that by always propagating the site method to extra downloads,
but only when they are specified as relative to the main download URI.

For the extra downloads that specify a full URI, it is not systematic
that it is the same site method. For example, a main download could be a
git clone, but an extra download a pure http download; in that case we
can't replicate the site method for extra downloads, so they'll have to
take appropriate care to specify the required method and encoding if
needed.

Reported-by: Jemy Zhang <jemy.zhang@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Jemy Zhang <jemy.zhang@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>