Alist of all packages available for activation.
Each element has the form (PKG . DESC), where PKG is a package
name (a symbol) and DESC is a vector that describes the package.
The vector DESC has the form [VERSION-LIST REQS DOCSTRING].
VERSION-LIST is a version list.
REQS is a list of packages required by the package, each
requirement having the form (NAME VL) where NAME is a string
and VL is a version list.
DOCSTRING is a brief description of the package.
This variable is set automatically by `package-load-descriptor',
called via `package-initialize'. To change which packages are
loaded and/or activated, customize `package-load-list'.