Alist of built-in packages.
The actual value is initialized by loading the library
`finder-inf'; this is not done until it is needed, e.g. by the
function `package-built-in-p'.
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.