pkg-generic: add a subdirectory to the DL_DIR

pkg-generic: add a subdirectory to the DL_DIR

With all the previous changes, we are now ready to add a subdirectory to
the DL_DIR.
The structure will now be DL_DIR/PKG_NAME/{FILE1,FILE2}

This is needed for multiple reasons:
    - Avoid patches with name like SHA1.patch laying flat in DL_DIR,
    which makes it hard to know to which packages they apply
    - Avoid possible collisions if two releases have the same name
    (e.g: v01.tar)
    - Allow the possibility to handle a git cache per package in the
    newly created subdirectory.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>