package/Makefile & ipkg-make-index.sh: add full package data list

The external script used to generate the package lists for the
LEDE wiki's table of packages [1] and package indexes [2] requires
a "Source:" field in the package lists to find package makefiles.
The package makefiles are used to read the package's Category and Submenu.

The "Source:" field was removed in commit
b4aa3c89
to reduce package list sizes and lessen opkg issues in low ram devices.

Add a separate package list file with full data to be used by the wiki's script.
It's called Packages.manifest and isn't compressed as it's not necessary.

1. https://lede-project.org/packages/start
2. https://lede-project.org/packages/index/start



Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>